D8307: Make Qt5::X11Extras really optional

[Kevin Funk]

kfunk added a reviewer: sitter.

REPOSITORY
  R871 DrKonqi

REVISION DETAIL
  https://phabricator.kde.org/D8307

To: kfunk, rjvbb, sitter
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart