[quake3] Make clean

[Ryan C. Gordon]

> I've been using cons, as per the README, got a clean build without needing
> to hack the source like the Makefile did; and if cons is anything like SCons,
> you shouldn't need to ever run clean to get a correct build :)

We should probably pick one or the other and remove the one we don't use.

The cons system isn't building the SDL stuff, btw, since it won't define 
USE_SDL=1 and doesn't have sdl_snd.c/sdl_glimp.c in the build.

--ryan.