ENH: Use pygccxml snapshot.

The pygccxml code has been moved from mercurial to git, and
is now hosted here: https://github.com/gccxml/pygccxml
ITK is still using version 1.0.0 until we have a stable
pygccxml version working with Python 2 and 3.

Changes:

1) Remove pygccxml folder from Wrapping/Generators/SwigInterface/
2) Pass the paths to pygccxml Snapshot and gccxml to the idx and igenerator module
3) pygccxml.parser.config.config_t() is deprecated and is now replaced by
pygccxml.parser.config.gccxml_configuration_t()
4) Removed some commented out and unused code, and moved the find_package(PythonInterp REQUIRED)
to the top of the SwigInterface CMakeLists.txt
5) Rename CABLE_INDEX variable to IDX_GENERATOR

Change-Id: I489675b506190ecc8f99f8258c33b09e03697a71
Merge topic 'UsepygccxmlSnapshot'

deccbdc1 ENH: Use pygccxml snapshot.