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

docs/README-cmake.md

@@ -105,7 +105,7 @@ When using a recent version of CMake (3.14+), it should be possible to:
 
 - build SDL for iOS, both static and dynamic
 - build SDL test apps (as iOS/tvOS .app bundles)
-- generate a working SDL_config.h for iOS (using SDL_config.h.cmake as a basis)
+- generate a working SDL_build_config.h for iOS (using SDL_build_config.h.cmake as a basis)
 
 To use, set the following CMake variables when running CMake's configuration stage: