+ Fix libvpx and zrtp compilation for XCode 4.x
Overall, we transition from using gcc to using clang all the way. This solves some tricky problems with -mtune options and architecture-dependant configure issues.
CMakeFileLists.txt was overwritten for each architecture target.
CMake was then confused as the CMakeFileLists.txt was newer than the
generated Makefile, cache.
CMake would forget the options passed during generation of Makefile,
leading to compilation of unwanted ccRrtp.