Warn Visual C++ users if they have a stale configure generated SDL_config.h

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402491

include/SDL_config.h.in

@@ -28,6 +28,11 @@
 /* General platform specific identifiers */
 #include "SDL_platform.h"
 
+/* Make sure that this isn't included by Visual C++ */
+#ifdef _MSC_VER
+#error You should copy include/SDL_config.h.default to include/SDL_config.h
+#endif
+
 /* C language features */
 #undef const
 #undef inline