Bring FindFontconfig.cmake up to ECM standards

Summary:
This is a preparation step to move this to ECM (or possibly CMake),
which makes sense now that qtbase is using this too (for Qt6).

Reviewers: #build_system, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: cgiboudeaux, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17547