rewrote bam.lua to be cleaner

bam invocation changed: bam compiler=[cl,gcc] arch=[x86,x86_64,ppc,ppc64] conf=[release,debug] [game,server,client,masterserver,versionserver,tools]
using pkg-config for SDL instead of sdl-config if available because of crosscompiling issues
by default binaries will only built for the current architecture on Mac OS X now, like on all other systems

The Windows & Linux parts have been tested, the Mac OS X part unfortunately not. If you have a Mac please try it.