COMP: Allow "external" compilation of TractographyLabelMapSeeding

Update CMakeLists.to reference variable also set in SlicerConfig.cmake

This should help to resolve the issue reported on the mailing list:
http://slicer-devel.65872.n3.nabble.com/Building-CLI-outside-of-the-Slicer-Build-tree-tt4031199.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23002 3bd1e089-480b-0410-8dfb-8563597acbee