Added SDL_mixer sound support compile conditions.

Default sound support library is now SDL_mixer. The CMake "SOUND"
directive controls what sound library the game will be compiled with.
When "-DSOUND=FMOD" is specified the sound lib chosen will be FMOD.
Added SDL_mixer initialization routines.

In case the "WITH_SDLMIXER" flag is passed the CSdl class will now
initialize SDL_mixer as sound support engine.
SDL_INIT_AUDIO moved to SDL sub system call.