Fix build with cmake < 3.7

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.

Differential Revision: https://phabricator.kde.org/D19581

(cherry picked from commit a08e2ebd589fc3faaa4b79f48fc7ff964e13cad3)