COMP: Configure ITKv4 with python executable

If on the build machine there is a python installed on the system, this one
will be found by the ITKModuleHeaderTest.cmake module. To keep the
SlicerConfig.cmake sanity checks quiet, this commit ensure ITKv4 will be
configured with the same python executable accross the entire Slicer build
tree.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23956 3bd1e089-480b-0410-8dfb-8563597acbee