COMP: Gracefully fail if Slicer build against an external non-system python

This commit relaxes the check initially introduced by commit r21191

Note that ideally the build system should be updated to install python
libraries and module even if they are not built into the usual
directory <Slicer-Superbuild>/python-install

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22943 3bd1e089-480b-0410-8dfb-8563597acbee