Introduce X11 "toolkit" and toolkitize message boxes

src/SDL_utils_c.h

@@ -78,4 +78,16 @@ extern char *SDL_CreateDeviceName(Uint16 vendor, Uint16 product, const char *ven
 // Log what backend a subsystem chose, if a hint was set to do so. Useful for debugging.
 extern void SDL_DebugLogBackend(const char *subsystem, const char *backend);
 
-#endif // SDL_utils_h_
+static SDL_INLINE int SDL_IntMax(int a, int b)
+{
+    return (a > b) ? a : b;
+}
+
+static SDL_INLINE int SDL_IntClamp(int a, int b, int c)
+{
+  if (a < b) return b;
+  if (a > c) return c;
+  return a;
+}
+
+#endif