dev-perl/PDL-Graphics-Gnuplot: Add use requirements for gnuplot, bug 622586

dev-perl/PDL-Graphics-Gnuplot: Add use requirements for gnuplot, bug 622586

Depending on the Gnuplot compilation options, differents sets of terminals are available. When (on Linux) either X or Qt4 terminals are enabled, one
of these automatically becomes default terminal after program start. This can be verified with "show terminal", and in this case also
PDL::Graphics::Gnuplot works fine.

However, if both X and Qt4 terminal are disabled during compilation, the default terminal after gnuplot start is "unknown", meaning it's not
functional (and a terminal has to be selected first). This makes the PDL::Graphics::Gnuplot tests fail, and probably also its functionality.

See also https://github.com/drzowie/PDL-Graphics-Gnuplot/issues/66

Package-Manager: Portage-2.3.6, Repoman-2.3.2