Xcode project: created "QuakeSpasmSDL2" target. It expects a copy of SDL2.framework to be in /Library/Frameworks

changes from "QuakeSpasm" target:
- remove double quotes around LIBRARY_SEARCH_PATHS values, this breaks linking on my OS X 10.6/Xcode 3.2.6 system
- drop ppc support
- add USE_SDL2 define
- bump minimum sdk deployment target to 10.5 as required by SDL2

Tested on OS X 10.6.8/Xcode 3.2.6, and OSX 10.9.4/Xcode 5.1.1.