GnuTLS - gmp.h: No such file or directory (5.4/testing/x64)

I have some compilation errors on 5.4/testing/x64. It worked properly some weeks ago. The culprit here seems gmp.
This is the log:

$ make -j1 package/gnutls/compile V=s
make[1]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile'
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/libs/toolchain'
echo "libc" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
echo "libgcc" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
echo "libatomic" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
echo "libstdcpp" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
echo "libpthread" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
echo "librt" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/toolchain.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/libs/toolchain'
time: package/libs/toolchain/compile#0.14#0.04#0.18
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/libs/nettle'
echo "libnettle" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/nettle.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/libs/nettle'
time: package/libs/nettle/compile#0.13#0.05#0.16
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/firmware/linux-firmware'
echo "bnx2-firmware" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux-firmware.default.install
echo "r8169-firmware" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux-firmware.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/firmware/linux-firmware'
time: package/firmware/linux-firmware/compile#0.19#0.05#0.21
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/firmware/prism54-firmware'
make[2]: Nothing to be done for 'compile'.
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/firmware/prism54-firmware'
time: package/firmware/prism54-firmware/compile#0.11#0.03#0.12
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/kernel/linux'
echo "kernel" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-aead" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-authenc" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-cmac" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-ecb" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-ecdh" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-hash" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-hw-padlock" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-kpp" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-manager" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-null" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
WARNING: kmod-crypto-pcompress is not available in the kernel config - generating empty package
echo "kmod-crypto-pcompress" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-crypto-user" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-hwmon-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-i2c-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-i2c-algo-bit" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-hid" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-input-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-input-evdev" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-lib-crc-ccitt" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-lib-crc16" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-libphy" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
WARNING: kmod-mii is not available in the kernel config - generating empty package
echo "kmod-mii" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-phy-realtek" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-r8169" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-e1000" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-e1000e" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-igb" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-bnx2" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-reject" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-reject6" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-ipt" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-ipt6" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ipt-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-conntrack" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-conntrack6" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-nat" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nf-flow" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ipt-conntrack" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ipt-offload" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ipt-nat" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ip6tables" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-slhc" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ppp" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-pppox" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-pppoe" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-nls-base" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-bluetooth" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-regmap-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-pps" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-ptp" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
echo "kmod-usb-core" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/linux.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/kernel/linux'
time: package/kernel/linux/compile#1.67#0.15#1.82
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/kernel/cryptodev-linux'
echo "kmod-cryptodev" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/cryptodev-linux.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/package/kernel/cryptodev-linux'
time: package/kernel/cryptodev-linux/compile#0.14#0.04#0.17
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/libtasn1'
echo "libtasn1" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/libtasn1.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/libtasn1'
time: package/feeds/packages/libtasn1/compile#0.15#0.05#0.18
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/p11-kit'
echo "p11-kit" >> /media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/pkginfo/p11-kit.default.install
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/p11-kit'
time: package/feeds/packages/p11-kit/compile#0.11#0.06#0.16
make[2]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/gnutls'
rm -f /media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/.built
touch /media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/.built_check
CFLAGS="-Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -ffile-prefix-map=/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12=gnutls-3.6.12 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro  -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include/fortify -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include " CXXFLAGS="-Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -ffile-prefix-map=/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12=gnutls-3.6.12 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro  -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include/fortify -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include " LDFLAGS="-L/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/usr/lib -L/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/lib -L/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/usr/lib -L/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/lib -znow -zrelro " make  -C /media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/. AR="x86_64-openwrt-linux-musl-gcc-ar" AS="x86_64-openwrt-linux-musl-gcc -c -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -ffile-prefix-map=/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12=gnutls-3.6.12 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro" LD=x86_64-openwrt-linux-musl-ld NM="x86_64-openwrt-linux-musl-gcc-nm" CC="x86_64-openwrt-linux-musl-gcc" GCC="x86_64-openwrt-linux-musl-gcc" CXX="x86_64-openwrt-linux-musl-g++" RANLIB="x86_64-openwrt-linux-musl-gcc-ranlib" STRIP=x86_64-openwrt-linux-musl-strip OBJCOPY=x86_64-openwrt-linux-musl-objcopy OBJDUMP=x86_64-openwrt-linux-musl-objdump SIZE=x86_64-openwrt-linux-musl-size CROSS="x86_64-openwrt-linux-musl-" ARCH="x86_64" ;
make[3]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12'
make  all-recursive
make[4]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12'
Making all in gl
make[5]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
make  all-recursive
make[6]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
Making all in .
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
make[7]: Nothing to be done for 'all-am'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
Making all in tests
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
make  all-recursive
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
Making all in .
make[9]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
make[9]: Nothing to be done for 'all-am'.
make[9]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl/tests'
make[6]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
make[5]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/gl'
Making all in lib
make[5]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib'
make  all-recursive
make[6]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib'
Making all in includes
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/includes'
make[7]: Nothing to be done for 'all'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/includes'
Making all in x509
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/x509'
make  all-am
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/x509'
make[8]: Nothing to be done for 'all-am'.
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/x509'
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/x509'
Making all in auth
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/auth'
make[7]: Nothing to be done for 'all'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/auth'
Making all in ext
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/ext'
make[7]: Nothing to be done for 'all'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/ext'
Making all in algorithms
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/algorithms'
make[7]: Nothing to be done for 'all'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/algorithms'
Making all in extras
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/extras'
make[7]: Nothing to be done for 'all'.
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/extras'
Making all in accelerated
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated'
Making all in x86
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated/x86'
make[8]: Nothing to be done for 'all'.
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated/x86'
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated'
make[8]: Nothing to be done for 'all-am'.
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated'
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/accelerated'
Making all in unistring
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
make  all-recursive
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
make[9]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
make[9]: Nothing to be done for 'all-am'.
make[9]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/unistring'
Making all in nettle
make[7]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/nettle'
make  all-am
make[8]: Entering directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/nettle'
x86_64-openwrt-linux-musl-gcc -DHAVE_CONFIG_H -I. -I../..  -I./int -I./backport -I./../../gl -I./../../gl -I./../includes -I./../includes -I./../../gl -I./..  -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/target-x86_64_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/usr/include -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include/fortify -I/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/include   -Os -pipe -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -ffile-prefix-map=/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12=gnutls-3.6.12 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro  -MT curve448/eccdata-eccdata.o -MD -MP -MF curve448/.deps/eccdata-eccdata.Tpo -c -o curve448/eccdata-eccdata.o `test -f 'curve448/eccdata.c' || echo './'`curve448/eccdata.c
curve448/eccdata.c:43:10: fatal error: gmp.h: No such file or directory
   43 | #include <gmp.h>
      |          ^~~~~~~
compilation terminated.
make[8]: *** [Makefile:2061: curve448/eccdata-eccdata.o] Error 1
make[8]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/nettle'
make[7]: *** [Makefile:1719: all] Error 2
make[7]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib/nettle'
make[6]: *** [Makefile:2202: all-recursive] Error 1
make[6]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib'
make[5]: *** [Makefile:1825: all] Error 2
make[5]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/lib'
make[4]: *** [Makefile:1598: all-recursive] Error 1
make[4]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12'
make[3]: *** [Makefile:1525: all] Error 2
make[3]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12'
make[2]: *** [Makefile:251: /media/LinuxPart2/OpenWRT/openwrt-compile/build_dir/target-x86_64_musl/gnutls-3.6.12/.built] Error 2
make[2]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile/feeds/packages/libs/gnutls'
time: package/feeds/packages/gnutls/compile#0.52#0.10#0.60
make[1]: *** [package/Makefile:113: package/feeds/packages/gnutls/compile] Error 2
make[1]: Leaving directory '/media/LinuxPart2/OpenWRT/openwrt-compile'
make: *** [/media/LinuxPart2/OpenWRT/openwrt-compile/include/toplevel.mk:227: package/gnutls/compile] Fehler 2

There are more broken packages (avahi, busybox/grep...) but these might be related to gmp as well i guess.

Any ideas why gmp.h is not found?
There is staging_dir/host/include/gmp.h and build_dir/host/gmp-6.2.0 (with ./.built file). Looks ok to me.

Arch Linux 5.5.11
OpenWrt master 556b8581a15c855b2de0efbea6b625ab16cc9daf
Testing kernel 5.4
binutils 2.34.1
gcc 9.3.0

Edit:
If i add/enable the libgmp package many of this errors come up. And they don't vanish when i disable libgsm again.

/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/lib/gcc/x86_64-openwrt-linux-musl/9.3.0/../../../../x86_64-openwrt-linux-musl/bin/ld: curve448/eccdata-eccdata.o: in function `ecc_init':
eccdata.c:(.text+0xa): undefined reference to `__gmpz_init'
/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/lib/gcc/x86_64-openwrt-linux-musl/9.3.0/../../../../x86_64-openwrt-linux-musl/bin/ld: eccdata.c:(.text+0x15): undefined reference to `__gmpz_init'
/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/lib/gcc/x86_64-openwrt-linux-musl/9.3.0/../../../../x86_64-openwrt-linux-musl/bin/ld: curve448/eccdata-eccdata.o: in function `ecc_set_str':
eccdata.c:(.text+0x34): undefined reference to `__gmpz_set_str'
/media/LinuxPart2/OpenWRT/openwrt-compile/staging_dir/toolchain-x86_64_gcc-9.3.0_musl/lib/gcc/x86_64-openwrt-linux-musl/9.3.0/../../../../x86_64-openwrt-linux-musl/bin/ld: eccdata.c:(.text+0x49): undefined reference to `__gmpz_set_str'