find cantor via config (and fall back to finder when not available)

Summary:
this is fully backwards compatible but has the advantage that it

• shows up in cmake feature summary
• will use the config when it is available (which is at least since 19.04)

Test Plan: still builds

Reviewers: asemke

Reviewed By: asemke

Subscribers: asemke, cfeck, apol, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D20808