Fix CMakeLists.txt to work with older versions of CMake

Summary: The `VERSION_GREATER_EQUAL` macro was only introduced with version 3.7, and many distros (including KDE Neon) ship an older version of CMake. This patch restores compatibility with those older versions.

Test Plan: Now I can build Discover again

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14112