avoid -Wundef warning from SDL_config_windows.h
diff --git a/include/SDL_config_windows.h b/include/SDL_config_windows.h
index bed9d42..1c838f8 100644
--- a/include/SDL_config_windows.h
+++ b/include/SDL_config_windows.h
@@ -195,7 +195,7 @@ typedef unsigned int uintptr_t;
#endif
/* Check to see if we have Windows 10 build environment */
-#if _MSC_VER >= 1911 /* Visual Studio 15.3 */
+#if defined(_MSC_VER) && (_MSC_VER >= 1911) /* Visual Studio 15.3 */
#include <sdkddkver.h>
#if _WIN32_WINNT >= 0x0601 /* Windows 7 */
#define SDL_WINDOWS7_SDK