This uses the FindPhonon.cmake file from kdelibs to find
the phonon package. Previously it would not build and gave a
somewhat confusing 'Undefined Interface' error when running moc
(cherry picked from commit 59ff91c95d5cb447c89bfdb8be02b518a136c7f6)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>