COMP: Ensure relaxed Qt version checking works for extension

Commit 21727 introduced the variable Slicer_EXPERIMENTAL_QT_VERSION, the
problem was that the variable wasn't added to SlicerConfig, this was causing
problem with the macro checking qt version within extension.

Thanks to Matt McCormick for reporting the issue.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21743 3bd1e089-480b-0410-8dfb-8563597acbee