Sandro Knauß authored Dec 13, 2018 and Ben Cooksley committed Dec 13, 2018

Specifically:
- kpeople uses KPeople instead of KF5People in filenames for cmake: fixes the search path.
- use Sets instead of Lists, as we don't want multiple entries anyways.
- no general include direcory is allowed to enter the header section, as those are not part of public API.
- add more known cmake parameters (purpose, kpeople).

Differential Revision: https://phabricator.kde.org/D17534