configure.ac.in: update library dependencies for gtk-3.x and xfce-4.14.

configure.ac.in: update library dependencies for gtk-3.x and xfce-4.14.

We have four obsolete dependencies that need to be updated to bring
xfce-hdaps into the future:

  * gtk+
  * libxfce4ui
  * libxfce4util
  * libxfce4panel

These are detected by the configure script which then does whatever it
needs to do to ensure that they get linked in the right places. Instead
of requesting gtk+-2.20.0 and the xfce-4.8.0 versions of the libraries, we
now request gtk+-3.0 and the xfce-4.14.0versions. This should immediately
break the build and give me a nice long list of tasks that need to be
completed to finish the migration.