[quake3] Make clean

[Jamie Wilkinson]

This one time, at band camp, Ryan C. Gordon wrote:
>
>>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.

Should be pretty trivial to add sdl_snd.c, etc. to the build in the
Conscripts, and add a USE_SDL option.