A full featured groff is required during buildworld, so build it always
and don't rely on it being present on the host system.
vgrind(1) is tightly coupled to a roff processor and will not be
built/installed when groff is disabled. Also much of the roff'ed
documentation under share/doc will not be built/installed when
WITHOUT_GROFF is defined.
Reviewed by: ru (partial)
