Use the standard _WIN32 instead of the custom MSWIN32 in libgd

We're already checking `_WIN32` elsewhere in our bundled libgd, so it
makes no sense to also have a custom `MSWIN32`.

Closes GH-7682.

Author: cmb69

ext/gd/config.w32

@@ -69,7 +69,6 @@ if (PHP_GD != "no") {
 /D HAVE_LIBPNG  \
 /D HAVE_XPM  \
 /D HAVE_GD_GET_INTERPOLATION \
-/D MSWIN32 \
 		");
 		if (ICC_TOOLSET) {
 			ADD_FLAG("LDFLAGS_GD", "/nodefaultlib:libcmt");

ext/gd/libgd/gdft.c

@@ -12,7 +12,7 @@
 #include "gd.h"
 #include "gdhelpers.h"
 
-#ifndef MSWIN32
+#ifndef _WIN32
 #include <unistd.h>
 #else
 #include <io.h>