Since commit 1786b121 (OS X: Allow CMAKE_OSX_SYSROOT to be a logical SDK
name, 2012-09-21) we support names like "macosx" or "macosx10.7" as the
specified value of CMAKE_OSX_SYSROOT.  Extend the SDK name->path
conversion to save the original value and also convert into a temporary
variable for the Xcode generator.  Re-implement the deployment target
sanity check to detect the version from the transformed path.