COMP: Fix MacOSX build errors following integration of CMake'ified python

The build errors introduced by commit r21863 and reported below
should now be fixed.

// -------------
Build errors:

[...]

dyld: Library not loaded: libpython2.7.dylib
  Referenced from: /Users/kitware/Dashboards/Nightly/Slicer-build-64bits-QT4.7.4-PythonQt-With-Tcl-CLI-Release-nightly/VTK-build/bin/vtkWrapPython
  Reason: image not found
[...]

Source: http://trunk.cdash.org/viewBuildError.php?buildid=82253
// -------------

Since the system where I did my test was having version 2.7.2 of python
installed in the system, the vtkWrapPython executable was resolving its
symbols against the system python and I didn't catch this error while
doing my test. On the other hand, the MacOSX factory coming with
version 2.6.1 of python, this particular problem occurred in that
specific case.

On Windows, the problem didn't happen because the DLL lives along
side the executable and on Linux the problem didn't happen because
the executable vtkWrapPython livres in the VTK build tree and was
embedding the location of the library using RPATH.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21867 3bd1e089-480b-0410-8dfb-8563597acbee