Use SDL_FALLTHROUGH if available

The SDL_FALLTHROUGH macro has been added to SDL 2.0.18. Until 2.0.18 is
released, use it if it's available.

desktop_version/src/TempSDLFallthrough.h

@@ -0,0 +1,22 @@
+#ifndef TEMPSDLFALLTHROUGH_H
+#define TEMPSDLFALLTHROUGH_H
+
+/* FIXME: Delete this when SDL 2.0.18 releases! */
+
+#include <SDL_stdinc.h>
+
+#ifndef SDL_FALLTHROUGH
+#   ifndef __has_attribute
+#      define __has_attribute(x) 0
+#   endif
+
+#   if __has_attribute(__fallthrough__)
+#      define VVV_fallthrough __attribute__((__fallthrough__))
+#   else
+#      define VVV_fallthrough do { } while (false) /* fallthrough */
+#   endif
+
+#   define SDL_FALLTHROUGH VVV_fallthrough
+#endif
+
+#endif /* TEMPSDLFALLTHROUGH_H */