gnuplot: new package

[Thomas: Improved by adding support to use readline functionality, gd
functionality, and various fixes.]

Signed-off-by: Anthony Viallard <viallard@syscom-instruments.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>