Check for python-cairo
Test Plan:
Works correctly when installed
Changed cmake locally to "import asdfasdf"
Got the correct error message and an aborted build
Reviewers: Plasma, ngraham, cgiboudeaux, zzag
Reviewed By: Plasma, ngraham, zzag
Subscribers: ngraham, zzag, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D18244