Fix the python setup in externals packages, 1.0 branch (2017.05.31.)

Made sure that whichever package looks for either PythonInterp or PythonLibs, would from now on look for both. To make sure that the resulting project would either not set up python at all, or would set it up properly.

In the latest, 1.0.13 tag multiple externals projects got broken because only PythonLibs was set up in them, without PythonInterp.

This is to fix the issue described in ATLINFR-1769. After the new version of the code is pulled into 21.2 (and eventually 21.0).

Jenkins Build SUCCESS

Results available at: Jenkins [AE-MERGE-REQUEST #77]

Let's fast-forward this...

merged

mentioned in commit 2551a6c0

mentioned in commit 5d9b4d44

mentioned in merge request !57 (merged)