#472 Add configure option to include support for ReGIS terminal

[Ulrich Müller]

Forwarding Gentoo bug https://bugs.gentoo.org/643888 (reported by Nikita Zlobin):

Some terminals, e.g. mlterm and terminology, have support for ReGIS in-terminal vector graphics. Gnuplot includes code to support this, but so far there is no easy way to enable it.

Attached patch adds a configure option to include ReGIS terminal support.

[Ethan Merritt]

OK.
Note, however, that the regis driver code in gnuplot has not been updated since 2004 and doesn't support many of the features and plot modes that have been added over the past 20 years. If you seriously want gnuplot in-terminal output I suggest trying the sixel terminal or dom terminals, which are both fully up to date and far more feature-full than regis ever was. I would guess that any terminal emulator that supports ReGIS probably also supports sixel graphics. The dom terminal (requiring in-terminal svg support) is a bit more obscure but still worth looking into if that's what you need.