For example if one installs Xcode 4.6 on OS X 10.9, it doesn't contain
a 10.9 SDK, so fallback to the next newest version which, in this case,
happens to be a 10.8 SDK.
This fixes bug #14572.
Initialize variables CMAKE_OSX_SYSROOT, CMAKE_OSX_DEPLOYMENT_TARGET, and
CMAKE_OSX_ARCHITECTURES prior to enabling any languages. This will
allow compiler identification to consider these values.