Autodetection for the pngcairo terminal of gnuplot.

Closes #322.
This patch  adds an autodetection if we can use gnuplot's pngcairo terminal (produces much better graphics and supports all linestyles the next maxima version will).

This autodetection is quite fast (50 miliseconds on my laptop). Since it seems there is no portable way of executing shell commands from lisp I delegated this task to maxima (since maxima has to call gnuplot it always will have a way to execute shell commands) and let maxima's compiler produce the necessary lisp files for me.