Use MinGW32 types for pthread_t and pthread_mutex_t

uintptr_t should be the same as a DWORD on Win32,
and CRITICAL_SECTION can be stored in an inptr_t as well.

This should work fine, and allow more build environments to be happy.
This is generally caused by new C++ standards having thread support
in the standard library, so wrappers getting pushed deeper in.

At some pointer, this should be redone with standard threads, but
now is not the time.