Clean up C flags (Dingux, GCW Zero)

Most of what is already covered by -O3 is deleted.

-fpeel-loops is deleted due to peeled loops being very large and
fitting less code in the instruction cache.
Use link-time optimisation (GCW Zero)

This reduces the size of the executable.
Do not link to SDL_mixer (Dingux, GCW Zero)

Those platforms don't use the menu music in .mod format.