[Groff] Add --with-doc configuration option

[Steffen Nurpmeso]

Hello,

well ok, i think i have something that i could post (after some
more checking, i.e., tomorrow).
What would you say about an additional --with-examples?
We could solely skip PDF_PROGRAMS if neiter is desired, then.
And how fully blown do you actually want it: should a `make dist'
fail and complain if documentation (plus) is not complete, or is
it ok to simply assume someone who runs `make dist' knows about
the make system?
Ciao,

--steffen