/usr/bin/ld: cannot find -lpthread

Yeah it is. But there's no libpthread.so in /lib/ which I'm guessing there should be, so I tried uninstalling it (forced uninstall, because it's an essential package) and re-installed with opkg hoping it would add the .so file, but it didn't. So I've been searching for an answer but no luck so far. How can I create the .so file if it doesn't come with the package in opkg?