[Gnuae] added gtkplot back in

[Rob Savoye]

  Now that GNOME is supporting the GTK Extra package (which contains gtkplot),
I turned on the configure support to look for that package, and to turn on
support in gpv/src for gtkplot support. For a long time it's been broke, and
only available via GNOME CVS, so I turned it off. I found some decent alegbra
tools, so I'm planning on bringing the whole Sandia calculations on PV
performance back to life. The calculations are boring without 3D plotting. :-)

        - rob -