At the superbuild level, it's now possible to pass ITK_DIR, CTK_DIR
or VTK_DIR/VTK_SOURCE_DIR.

The developer has to make sure the directory match the expected
configuration (wrapping, options ... )

This is useful when feature are developed locally on either CTK / VTK  / ITK
and the developer doesn't want to commit to a repository each time
he would like to see he obtain the expected behavior within slicer.

Variable name, build dir, source dir are also now identified using the same
prefix.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@13780 3bd1e089-480b-0410-8dfb-8563597acbee