Make SDL_stdinc.h work when compiling with -Wundef (thanks, Ben!). Fixes Bugzilla #2664.
diff --git a/include/SDL_stdinc.h b/include/SDL_stdinc.h
index 7cf0208..2a0aba4 100644
--- a/include/SDL_stdinc.h
+++ b/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)