D23551: CMake config files: use as min dep version the Qt version we built against

[Friedrich W. H. Kossebau]

kossebau added a reviewer: Build System.

REPOSITORY
  R263 KXmlGui

REVISION DETAIL
  https://phabricator.kde.org/D23551

To: kossebau, #frameworks, aacid, #build_system
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns