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
removed unnecessary objs directory
generated files are now in build/ as well
also fixed python invocation on windows again
fix compiler warnings

from 191feb618f
added clang support
Merge pull request #1072 from minus7/new-bam-file

rewrote bam.lua to be cleaner