D28511: Fix for older cmake versions

[Bhushan Shah]

bshah accepted this revision.

REPOSITORY
  R108 KWin

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D28511

To: apol, #kwin, bshah, davidedmundson
Cc: kwin, Orage, cacarry, LeGast00n, The-Feren-OS-Dev, cblack, jraleigh, zachus, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, romangg, jensreuterberg, abetts, sebas, apol, ahiemstra, mart
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