Another FLTK/gnuplot divergence

[Rik]

8. FLTK also can't do printing of hidden figures.

It would be nice to switch the production of figures for the Octave manual
from gnuplot to FLTK now that it is the default.  The only problem is that
the window will have to flash on the screen and close as the make process
is going on.  This shouldn't happen very often as the images are only
re-built after a 'make maintainer-clean' and they are distributed with the
documentation so newbies won't have to see this.

Any objections to making this change?

--Rik