Fix warnings on Windows.

Avoid macro redefinition warnings.

Reported-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAApHDvocHp4SXcPeMTwFiCQGaf9JypjTJ3Bh90jcPuGwxyDjjQ%40mail.gmail.com

src/include/port/win32_port.h

@@ -87,7 +87,9 @@
  * We have a replacement for fdatasync() in src/port/fdatasync.c, which is
  * unconditionally used by MSVC and Mingw builds.
  */
+#ifndef HAVE_FDATASYNC
 #define HAVE_FDATASYNC
+#endif
 
 #define USES_WINSOCK