COMP: Fix configuration of Slicer against system python

When building against system python, the pydicom module won't be
installed at configure/build time.

A Slicer runtime test could be added to check if expected python module
are available.

Thanks to "Dominique Belhachemi <domibel@debian.org>" for reporting the
problem.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22822 3bd1e089-480b-0410-8dfb-8563597acbee