This is mingw binary replacement of pthread library, so you can compile programs for old Windows with newest msys2/mingw-w64.
Compile (make) and add link flags to your program:
<path-to-pthread9x>/crtfix.o -static -L<path-to-pthread9x> -lpthread
-lpthread should be at first place, so GCC use symbols from this lib in first place.
Copy default.mk to config.mk. See this file for more information.
Original sources is from older mingw release (winpthreads), TryEnterCriticalSection code is from KernelEx code.