ENH: SlicerConfig: Facilitate building of extension adding VCS command

This commit will make it easier to locally build extension by simply
providing -DSlicer_DIR:PATH=/path/to/Slicer-Superbuild-Release/Slicer-build

Note that on MacOSX, CMAKEOSX* option still need to be provided.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25523 3bd1e089-480b-0410-8dfb-8563597acbee