gnuplot: configure GDFONTPATH using 'fontconfig'

This patch adds a wrapper script around gnuplot that uses fontconfig's
fc-list(1) utility to determine the set of available fonts and makes that list
available to Gnuplot in the $GDFONTPATH variable.

pkgs/tools/graphics/gnuplot/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, zlib, gd, texinfo
+{ stdenv, fetchurl, zlib, gd, texinfo, makeWrapper
 , texLive ? null
 , lua ? null
 , emacs ? null
@@ -11,8 +11,11 @@
 , cairo ? null
 , pkgconfig ? null
 , readline
+, fontconfig ? null, gnused ? null, coreutils ? null
 }:
 
+assert libX11 != null -> (fontconfig != null && gnused != null && coreutils != null);
+
 stdenv.mkDerivation rec {
   name = "gnuplot-4.4.4";
 
@@ -21,13 +24,21 @@ stdenv.mkDerivation rec {
     sha256 = "1zfv3npsxfn743wl65ibh11djxrc8fxzi2mgg75ppy6m12fmja6j";
   };
 
-  configureFlags = if libX11 != null then ["--with-x"] else ["--without-x"];
-
   buildInputs =
     [ zlib gd texinfo readline emacs lua texLive libX11 libXt libXpm libXaw
-      wxGTK pango cairo pkgconfig
+      wxGTK pango cairo pkgconfig makeWrapper
     ];
 
+  configureFlags = if libX11 != null then ["--with-x"] else ["--without-x"];
+
+  postInstall = stdenv.lib.optionalString (libX11 != null) ''
+    wrapProgram $out/bin/gnuplot \
+       --prefix PATH : '${gnused}/bin' \
+       --prefix PATH : '${coreutils}/bin' \
+       --prefix PATH : '${fontconfig}/bin' \
+       --run '. ${./set-gdfontpath-from-fontconfig.sh}'
+  '';
+
   meta = {
     homepage = "http://www.gnuplot.info";
     description = "A portable command-line driven graphing utility for many platforms";

pkgs/tools/graphics/gnuplot/set-gdfontpath-from-fontconfig.sh

@@ -0,0 +1,4 @@
+p=( $(for n in $(fc-list | sed -r -e 's|^([^:]+):.*$|\1|'); do echo $(dirname "$n"); done | sort | uniq) )
+IFS=:
+export GDFONTPATH="${GDFONTPATH}${GDFONTPATH:+:}${p[*]}"
+unset IFS p