D14112: Fix CMakeLists.txt to work with older versions of CMake

[Nathaniel Graham]

ngraham created this revision.
ngraham added reviewers: apol, Discover Software Store.
Restricted Application added a project: Plasma.
Restricted Application added a subscriber: plasma-devel.
ngraham requested review of this revision.

REVISION 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

REPOSITORY
  R134 Discover Software Store

BRANCH
  Plasma/5.12

REVISION DETAIL
  https://phabricator.kde.org/D14112

AFFECTED FILES
  libdiscover/backends/PackageKitBackend/CMakeLists.txt

To: ngraham, apol, #discover_software_store
Cc: plasma-devel, ragreen, Pitel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol, mart
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