Remove SDL_config.h from the public headers

The SDL headers are no longer dependent on the build configuration.

Fixes https://github.com/libsdl-org/SDL/issues/6643 and https://github.com/libsdl-org/SDL/issues/6641

test/testshape.c

@@ -10,8 +10,7 @@
   freely.
 */
 #include <stdlib.h>
-#include <math.h>
-#include <stdio.h>
+
 #include "SDL.h"
 #include "SDL_shape.h"