don't set python exe to undefined
Summary:
FindPythonInterp sets PYTHON_EXECUTABLE, we overrode it by setting an
empty var. add a comment to explain the unbalancedness of the conditional
branches.
Test Plan: pythoncairo is correctly found with cmake 3.10.2
Reviewers: davidedmundson, fvogt
Reviewed By: fvogt
Subscribers: fvogt, heikobecker, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18309