Avoid overwriting Qt5_FOUND with Qt5DBus or Qt5WebEngine disabled

Summary: Fixes at least a big cmake warning and potentially ppc64 build.

Test Plan: Builds fine with and without Qt5WebEngine.

Reviewers: #marble, rahn

Reviewed By: #marble, rahn

Subscribers: rahn, marble-devel, kde-edu

Tags: #marble, #kde_edu

Differential Revision: https://phabricator.kde.org/D18141