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

src/cpuinfo/SDL_cpuinfo.c

@@ -18,9 +18,7 @@
      misrepresented as being the original software.
   3. This notice may not be removed or altered from any source distribution.
 */
-#ifdef TEST_MAIN
-#include "SDL_config.h"
-#else
+#ifndef TEST_MAIN
 #include "../SDL_internal.h"
 #endif