Summary:
Flatpak has been patching this for a while, there doesn't seem to be a way to
use it there, so we better offer the option upstream.

Test Plan: Been building and using solid on flatpak runtimes for a while.

Reviewers: #frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: asturmlechner, davidedmundson, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D20230