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/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