COMP: Fix superbuild when using Xcode 5.0 or Xcode 5.0 command line tools

Xcode no longer ships with MacOSX10.7.sdk (or earlier) from version 5.0,
which is the most recent version specified in
CMake/SlicerBlockSetCMakeOSXVariables.cmake. Therefore the superbuild
fails when attempted to build Python and NumPy due to a missing version
number or SDK location. Manually setting CMAKE_OSX_DEPLOYMENT_TARGET
to 10.8 and CMAKE_OSX_SYSROOT to ${SDK_ROOT}/MacOSX10.8.sdk or
editing SlicerBlockSetCMakeOSXVariables.cmake fixes the issue.

Fixes #3402

From: Jamie Snape <jamie.snape@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22454 3bd1e089-480b-0410-8dfb-8563597acbee