#199 Replace ogg/vorbis lib dependency with stb_vorbis

[Dominik Reichardt]

SDL_sound recently replaced its dependency on Ogg/vorbis libs with a modded stb_vorbis https://hg.icculus.org/icculus/SDL_sound/rev/560bc96b53cc .
This might be something to do for Exult as well, it would further reduce the dependancies.
I'm adding Colourless as he was integral in getting rid of SDL_Mixer, so he might be interested in looking at this.