Make CFitsio optional

Summary: Thanks-to: rkflx

Test Plan: Built with and without cfitsio enabled.

Reviewers: gateau, ckertesz, rkflx

Reviewed By: gateau, ckertesz, rkflx

Subscribers: lbeltrame, mutlaqja, ckertesz, rkflx

Differential Revision: https://phabricator.kde.org/D8423