[Cmake] Using variables in CMakeLists.txt

[Amitha Perera]

Is it safe to use the name of a library to extract its path? For
example, in vxl, is it safe to do something like

   INCLUDE_DIRECTORIES( ${netlib} )
   INCLUDE( ${netlib}/CMakeListsLink.txt ) ?

These variables are marked "INTERNAL" in the cache. If it is safe for
the moment, how may things change when (if?) things change to put all
the libraries in a single directory?

Thanks,
Amitha.