pdflib-lite is required to enable Gnuplot's PDF terminal.
While not free (as in speach) software, pdflib-lite is available for
non-commercial (i.e., academic and personal) use free of charge. A license
reminder is printed at the end of the build.
pdflib is also provided by MacPorts; a workaround from their portfile was
adapted to this formula to get the build to go through.
