D23635: Fix build with cmake < 3.7

[Ralf Habacker]

habacker accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R139 Umbrello

REVISION DETAIL
  https://phabricator.kde.org/D23635

To: wbauer, #umbrello, habacker
Cc: umbrello-devel, vanderbilt
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