Remove support for mingw

MSVC works fine and the bundled mingw libraries are outdated.

src/arch/Threads/Threads_Pthreads.cpp

@@ -244,7 +244,7 @@ MutexImpl *MakeMutex( RageMutex *pParent )
 #if defined(UNIX)
 #include <dlfcn.h>
 #include "arch/ArchHooks/ArchHooks_Unix.h"
-#endif // On MinGW clockid_t is defined in pthread.h
+#endif
 namespace
 {
 	typedef int (* CONDATTR_SET_CLOCK)( pthread_condattr_t *attr, clockid_t clock_id );