Fix unnecessary cmake warnings (Qt5_FOUND to FALSE), use QT_REQUIRED_VERSION

Authored by asturmlechner on Feb 2 2019, 3:13 PM.

Description

Fix unnecessary cmake warnings (Qt5_FOUND to FALSE), use QT_REQUIRED_VERSION

Reviewers: Amarok, markey, heikobecker

Reviewed By: heikobecker

Subscribers: heikobecker, amarok-devel

Differential Revision: https://phabricator.kde.org/D18671