COMP: Fix MacOSX packaging explicitly setting VTK_LIBRARY_DIRS for VTK6

This is a regression that has probably been introduced
by kitware/VTK@0c1471f1

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23539 3bd1e089-480b-0410-8dfb-8563597acbee