D12674: Mark `Phonon4Qt5` dependency as optional in CMakeLists file

[Aleix Pol Gonzalez]

apol added a comment.


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REVISION DETAIL
  https://phabricator.kde.org/D12674

To: tundracomp
Cc: apol, ltoscano, cgiboudeaux, #frameworks, michaelh, ngraham, bruns