Updates to SDL_sound.

[Ryan C. Gordon]

> Erhm, the decoders already #included config.h (unless I forgot to
> commit this) via SDL_sound_internal.h

They do, but the #include SDL_sound_internal.h line is inside the #ifdef
SOUND_SUPPORTS_* check, so the files get discarded before they get to the
SDL_sound_internal.h inclusion.

--ryan.