Some FLAC decoder cleanups

[Ryan C. Gordon]

(Whoops; somehow this email fell through the cracks of my inbox; sorry for
the delay in replying...)

> I've changed the decoder to include FLAC/stream_decoder.h instead of
> FLAC/all.h. Does that mean that configure.in should be changed as well?

I made the change.

> I've removed the explicit test to see if the input appears to be FLAC
> data becuase I'm hoping that the check for metadata will automagically
> weed out all bad input. I'm not sure about this.

Cool; it's in CVS. Err...is it an error if a FLAC stream doesn't have any
metadata, or are there certain types of metadata that are required in a
valid FLAC stream?

> Finally, can anyone explain this transcript?
>
>      Sound_SetError("SHN: Not a SHN file");
>      [...]
>      Sound_SetError("FLAC__STREAM_DECODER_ERROR_LOST_SYNC");
>      Sound_SetError("FLAC__STREAM_DECODER_ERROR_LOST_SYNC");
>      Error in decoding sound file!
>        reason: [SHN: Not a SHN file].

Might be a problem with our use of SDL_SetError(), which is all
Sound_SetError() is right now. I'll take a look.

--ryan.