COMP: Force python build to Release

After updating Slicer build system to use CMake'ified python (r21863), Windows
debug build started to fail complaining that there was an "Invalid solution
configuration"

Since CMake'ified python currently force its build type to Release [1], this
explained the error message.

To make sure the command associated with python external project works
as expected, this commit overwrite the value CMAKE_CFG_INTDIR before
adding the python external project and restore it afterward.

[1] https://github.com/davidsansome/python-cmake-buildsystem/commit/3bcf841e28b255e62562e1dcc93a347bdbc78f1a

See http://na-mic.org/Mantis/view.php?id=3056
Fixes #3056

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21879 3bd1e089-480b-0410-8dfb-8563597acbee