Revert "Remove the deprecated PYTHON_INCLUDE_PATH variable."

This reverts commit 698f17d64b093db488de7d5ef33a8c15adfe8765.
The deprecated PYTHON_INCLUDE_PATH variable was still being
used by Wrapping/Python/CMakeLists.txt.

Change-Id: I3e9e433b974ffbbf38f5a546a95b526ccfda2581
Merge topic 'python-version-match' into master

530bc43e Revert "Remove the deprecated PYTHON_INCLUDE_PATH variable."