Updated Gnuplot to version 4.2.5.

* Merged 'gnuplot' and 'gnuplotX' attributes into one that uses (getPkgConfig
   "gnuplot" "x11" false) to determine whether X11 support is desired or not.

 * Added Emacs as a build-time dependency to ensure that Gnuplot's lisp code is
   installed, too.

 * Added optional support for the wxt terminal by means of (getPkgConfig
   "gnuplot" "wxGtk" false).

 * Removed the '-nix' suffix to the package name. The Gnuplot license requires
   a vendor-specific version suffix only in case of modified binary
   distributions. That is not the case here.

svn path=/nixpkgs/trunk/; revision=16992