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