VERSION_GREATER_EQUAL requires cmake 3.7 or higher, but the minimum version supported by KDE Frameworks is 3.5.
So use NOT VERSION_LESS instead, which is logically equivalent and also
works with older cmake versions.

Builds fine with cmake 3.5.2 now.
Before, I got this cmake error:

CMake Error at data/CMakeLists.txt:69 (if):
  if given arguments:

    "3.5.2" "VERSION_GREATER_EQUAL" "3.13.0"

  Unknown arguments specified

Still builds fine with newer cmake versions, tested with 3.10.2 and 3.13.4.