remove SDK

This will default to the current one (SDKROOT = macosx)
Removing GC target
Removing unneeded GC configurations
Setting Deployment Target 10.7 globally

(we don}t really care about PPC support, etc)
Clang errors when format strings expected
but aren’t provided
Build only active ARCH (at Xcode5 suggestion)
Need to turn this -Werror off too
Another Werror to remove