* include/winnt.h (MemoryBarrier): Define to __mingworg_MemoryBarrier()

which is in libmingwex.a.
	MODIFIED FROM: Use __sync_synchronize instead of
	creating a function due to multiple definition.
	NOTE: Not using __sync_synchronize to allow applications to mix-n-match
	C libraries from different vendors.

winsup/w32api/include/winnt.h

@@ -4041,7 +4041,8 @@ struct _TEB * NtCurrentTeb(void);
 # elif defined(_IA64_)
 #  define MemoryBarrier __mf
 # else
-#  define MemoryBarrier __sync_synchronize
+  void __mingworg_MemoryBarrier(void);
+#  define MemoryBarrier __mingworg_MemoryBarrier
 # endif
 #else
 # define MemoryBarrier