D22990: Fix build with older cmake

[Wolfgang Bauer]

wbauer created this revision.
wbauer added a reviewer: gottfried.
wbauer added a project: KDE Applications.
Herald added a project: KDE Edu.
Herald added a subscriber: kde-edu.
wbauer requested review of this revision.

REVISION SUMMARY
  Setting the cmake_policy fails if an older cmake version is used, so only set it to 3.10 if cmake is really 3.10 or higher.

TEST PLAN
  Building with cmake 3.5.2 succeeds again, before I got this error:
  
    CMake Error at CMakeLists.txt:11 (cmake_policy):
      An attempt was made to set the policy version of CMake to "3.10" which is
      greater than this version of CMake.  This is not allowed because the
      greater version may have new policies not known to this CMake.  You may
      need a newer CMake version to build this project.
  
  The warnings that appeared with cmake 3.10+ earlier (before 56ec6f23e9c4cc99c5c7d28f312b580755cbd285 <https://phabricator.kde.org/R336:56ec6f23e9c4cc99c5c7d28f312b580755cbd285>) are still gone.

REPOSITORY
  R336 KTouch

REVISION DETAIL
  https://phabricator.kde.org/D22990

AFFECTED FILES
  CMakeLists.txt

To: wbauer, gottfried
Cc: kde-edu, #kde_applications, fbampaloukas, alexeymin, genaxxx, jriddell, narvaez, apol
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