gnu: gtk+: Update to 3.16.0.

* gnu/packages/gtk.scm (gtk+): Update to 3.16.0.  Add libepoxy to
  propagated-inputs.  Add gettext to native-inputs.  Add a 'pre-configure'
  phase instead of modifying the existing 'configure' phase.  Fix references
  to 'gtk-update-icon-cache' in Makefile.in files.

Co-Authored-By: Mark H Weaver <mhw@netris.org>