USE_QT5=declarative né Qt quick1 has been put to rest upstream [1], and
is no longer shipped with Qt5 [that's why it has been kept at 5.5.1 while the
rest of Qt has been updated to 5.6.1].
Similar to arch [2] and debian [3] also get rid of it as a dependency for
qt5-phonon4.
[1] http://permalink.gmane.org/gmane.comp.lib.qt.releasing/1973
[2] https://git.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/phonon&id=1e16901aa3991d10e94906316258759be8e6b3d7
[3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=824957