[sdlsound] Added Ogg Opus decoding

[Kevin CROFT]

Graphical diff here:
https://gitlab.com/krcroft/sdl_sound/compare/aabf44b4...b6375357?expanded=1

It includes:
 - Opus decoding, using a circular decode buffer
 - Ogg to Vorbis change (per the fixme)
 - small improvements to autotool config
 - and gcc warning clean-ups

Runs clean in valgrind without memory issues or leaks. I've been using it
with Dosbox to mount CUE-CDDA audio disks composed of Opus-compressed
tracks + CUE file.

Happy to have someone kick the tires and provide a second set of eyes.

Regards,
 Kevin
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