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:
 

docs/README-migration.md

@@ -130,6 +130,7 @@ SDL_RWFromFP(void *fp, SDL_bool autoclose)
 
 ## SDL_stdinc.h
 
+The standard C headers like stdio.h and stdlib.h are no longer included, you should include them directly in your project if you use non-SDL C runtime functions.
 M_PI is no longer defined in SDL_stdinc.h, you can use the new symbols SDL_PI_D (double) and SDL_PI_F (float) instead.
 
 

docs/README-porting.md

@@ -23,10 +23,7 @@ There are two basic ways of building SDL at the moment:
 
 2. Using an IDE:
 
-   If you're using an IDE or other non-configure build system, you'll probably
-   want to create a custom SDL_config.h for your platform.  Edit SDL_config.h,
-   add a section for your platform, and create a custom SDL_config_{platform}.h,
-   based on SDL_config_minimal.h and SDL_config.h.in
+   If you're using an IDE or other non-configure build system, you'll probably want to create a custom `SDL_build_config.h` for your platform.  Edit `include/build_config/SDL_build_config.h`, add a section for your platform, and create a custom `SDL_build_config_{platform}.h`, based on `SDL_build_config_minimal.h` and `SDL_build_config.h.cmake`
 
    Add the top level include directory to the header search path, and then add
    the following sources to the project: