Source checkout properties are now extracted from the source directory
associated with the top-level CMake project.

This commit also simplifies how major/minor/patch version values are
passed to the inner build. Version variables are now always expected to
be Slicer_VERSION_(MAJOR,MINOR,PATCH).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26536 3bd1e089-480b-0410-8dfb-8563597acbee