This commit implements the check looking for required python modules
when building against system version of python.

For example, assuming that all required packages are installed in
a python virtual environment named "slicer":

$ cmake -DQT_QMAKE_EXECUTABLE:FILEPATH=/path/to/bin/qmake \
  -DCMAKE_BUILD_TYPE:STRING=Release \
  -DSlicer_USE_SYSTEM_python:BOOL=1 \
  ../Slicer

would output text like this one:

[...]
-- SuperBuild -         python-pyparsing => Requires python[INCLUDED],
-- SuperBuild -         python-pyparsing[OK] (SYSTEM)
-- PYPARSING_PATH: /home/jcfr/.virtualenvs/slicer/local/lib/python2.7/site-packages
-- PYPARSING_VERSION: 2.2.0
-- SuperBuild -       python-packaging[OK] (SYSTEM)
-- PACKAGING_PATH: /home/jcfr/.virtualenvs/slicer/local/lib/python2.7/site-packages/packaging
-- PACKAGING_VERSION: 16.8
-- SuperBuild -       python-six => Requires python[INCLUDED],
-- SuperBuild -       python-six[OK] (SYSTEM)
-- SIX_PATH: /home/jcfr/.virtualenvs/slicer/local/lib/python2.7/site-packages
-- SIX_VERSION: 1.10.0
[...]

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26055 3bd1e089-480b-0410-8dfb-8563597acbee