Compatibility with versions prior to 2.8.12 is being deprecated,
resulting in build warning messages. The minimum for the official
supported distros is 3.7.2 (Stretch), so the version could be moved
forward when newer features are required.
Reference: https://cmake.org/cmake/help/v3.19/release/3.19.html#deprecated-and-removed-features
One usage of the macro remains in the bundled libmygpo-qt5. This should
be updated once the pending upstream fix was merged. qtsingleapplication
is already fixed upstream.
