Allow SDL2 to be used for GTK3 builds

[jobbautista9]

Currently you're stuck with ALSA, PulseAudio, and JACK as options for sound backends when using the gtk3 version. I think the sound code for GTK3 should be replaced with one that uses SDL2 instead, which supports more sound backends like sndio.

[myon98]

Hi, thanks for taking interest. Lots of very questionable implementation choices in this project comes from me wanting to explore various APIs back then (otherwise why the hell should this have a horribly written custom D3D9/OpenGLES rendering engine for fmdsp??). I can't say when it will be implemented, but I do agree that having a SDL audio backend should be better.