COMP: Fix module library build macros to consider already set variables

This commit fixes the build of SlicerRt extensions.

It basically removes the hack introduced in r24530 (COMP: Set loadable
module _INCLUDE_DIRS variables in Slicer project) and fixes the build
of extensions already setting the <ProjectName>_INLCUDE_DIRS variable
in their Qt and VTK module library CMakeLists.txt.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24532 3bd1e089-480b-0410-8dfb-8563597acbee