COMP: External_SimpleITK - Fix unused CMake variable warnings

CMake Warning:
  Manually-specified variables were not used by the project:

    BUILD_EXAMPLES
    PYTHON_FRAMEWORKS

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22178 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Simplify passing of SimpleITK external project arguments

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22179 3bd1e089-480b-0410-8dfb-8563597acbee