Make SDL_stdinc.h work when compiling with -Wundef (thanks, Ben!).

Fixes Bugzilla #2664.

include/SDL_stdinc.h

@@ -226,7 +226,7 @@ typedef uint64_t Uint64;
 #define SDL_PRINTF_VARARG_FUNC( fmtargnumber )
 #define SDL_SCANF_VARARG_FUNC( fmtargnumber )
 #else
-#if _MSC_VER >= 1600 /* VS 2010 and above */
+#if defined(_MSC_VER) && (_MSC_VER >= 1600) /* VS 2010 and above */
 #include <sal.h>
 
 #define SDL_IN_BYTECAP(x) _In_bytecount_(x)