If the automatically selected SDK is newer than the host OS version
and no deployment version has been set then adjust the deployment
version to the host OS version. Otherwise the user won't be able to
launch the executables on the build host.
This is for example a problem on a MacOSX 10.10 host with Xcode 7
which only provides a MacOSX10.11 SDK.
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.