"No package 'protobuf' found" in v18.06.0-rc2

I tried to add the OpenConnect server and got this error:

checking for protobuf >= 2.6.0... no
configure: error: Package requirements (protobuf >= 2.6.0) were not met:

No package 'protobuf' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables protobuf_CFLAGS
and protobuf_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
Makefile:67: recipe for target '/home/user/git/openwrt/build_dir/hostpkg/libprotobuf-c-v1.2.1/.configured' failed
make[3]: *** [/home/user/git/openwrt/build_dir/hostpkg/libprotobuf-c-v1.2.1/.configured] Error 1
make[3]: Leaving directory '/home/user/git/openwrt/feeds/packages/libs/protobuf-c'
Command exited with non-zero status 2
time: package/feeds/packages/protobuf-c/host-compile#7.62#0.48#11.76
package/Makefile:107: recipe for target 'package/feeds/packages/protobuf-c/host/compile' failed
make[2]: *** [package/feeds/packages/protobuf-c/host/compile] Error 2
make[2]: Leaving directory '/home/user/git/openwrt'
package/Makefile:103: recipe for target '/home/user/git/openwrt/staging_dir/target-mipsel_74kc_musl/stamp/.package_compile' failed
make[1]: *** [/home/user/git/openwrt/staging_dir/target-mipsel_74kc_musl/stamp/.package_compile] Error 2
make[1]: Leaving directory '/home/user/git/openwrt'
/home/user/git/openwrt/include/toplevel.mk:216: recipe for target 'world' failed
make: *** [world] Error 2

It works after I selected the protobuf library manually and made sure libprotobuf-c was not selected. I could then reselect it and compile everything successfully. I assume this should be reported as a bug but I couldn't find out how from the reporting bugs section on the wiki.

I made a bug report here: https://github.com/openwrt/packages/issues/6522

I am still on OpenWrt 18.06.2 on my own build, but it still the same error.
How can I build it with OpenWrt 18.06.2 self compiled OpenWrt?

the same mistake how can I not fail again?