Shove all build system generated headers in include/build_config. Doing this, cmake does not need to copy the headers anymore. This was done for external projects vendoring SDL. See https://github.com/libsdl-org/SDL/issues/6641#issuecomment-1328092535
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