D19581: Fix build with cmake < 3.7

[Volker Krause]

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

REPOSITORY
  R216 Syntax Highlighting

REVISION DETAIL
  https://phabricator.kde.org/D19581

To: wbauer, vkrause
Cc: kwrite-devel, kde-frameworks-devel, cullmann, gennad, bmortimer, domson, michaelh, genethomas, ngraham, bruns, demsking, vkrause, sars, dhaumann
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