Updates to SDL_sound.

[Ryan C. Gordon]

> I see... but then why not just move the #include SDL_sound_internal.h
> out of that check?

I did, initially, but I figured that for simplicity in a file that may
become an empty stub (if SOUND_SUPPORTS_* is not defined), then it would
be best to put as little header spam as possible in there.
SDL_sound_internal.h needs SDL_sound.h to work correctly, and now there's
all sorts of stuff getting declared...I just assumed it's safer to include
the config.h directly, and only include SDL_sound*.h if needed.

But, of course, I'm going to go with your judgement on this; the current
state of CVS (which is easily reversable/changable) was mostly intended to
get things building here.

--ryan.