COMP: Use PATCH_COMMAND to set python project patch step

Using PATCH_STEP was preventing python from being configured, using
PATCH_COMMAND is the only documented way of associating a patch step with
an external project.

See http://www.cmake.org/cmake/help/v2.8.12/cmake.html#module:ExternalProject

See #3158

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22830 3bd1e089-480b-0410-8dfb-8563597acbee