To be fair, you'll probably have better luck finding people who can compile it if you use pthreads.h vs. threads.h. IIRC glibc doesn't have threads.h implemented, and I don't know of any C standard libraries that do. pthreads is fairly universal even if it's not standard C, and at the very least it's much more universal then threads.h is at the moment.
Correct, I was just going to chime in about this. Musl is very impressive with its compact size, correctness, Drepper-less-ness =P, and speed of implementing new standards.