Add support for threads.h from C11
Threading is a pretty fundamental feature for any modern program, and it sucks that there's really no good cross platform support for threading.
there's POSIX Threads, which are still a separate third party library, that windows does not support, and C11 really simplifies that by including it built into the language.
Really the only difficult part would be adding support for the tertiary features, like stdatomic.
A few standard libraries already support it, like musl, and PDCLib, but support isn't quite as common as it should be.