Work around MinGW-w64 erroneously claiming to have flockfile()

The _POSIX_THREAD_SAFE_FUNCTIONS constant is supposed to be defined only
if flockfile() and friends are available. MinGW-w64 defines that
constant, but the functions are not available. Work around that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.h

@@ -6,6 +6,12 @@ typedef _sigset_t sigset_t;
 #endif
 #include <winsock2.h>
 #include <ws2tcpip.h>
+
+/* MinGW-w64 reports to have flockfile, but it does not actually have it. */
+#ifdef __MINGW64_VERSION_MAJOR
+#undef _POSIX_THREAD_SAFE_FUNCTIONS
+#endif
+
 /*
  * things that are not available in header files
  */