because grotty-1.21 requires a new -c option which grotty-1.15 rejected.
Symptoms noticed by deraadt@ in mgp(1), Imake configuration spotted
as the culprit by espie@, patch tested and ok by matthieu@.
GL manuals because Imake still tried to use nroff(1), tbl(1), and eqn(1)
which do not exist in base any longer. Thus, let the Xenocara build use
mandoc(1) instead, which has native tbl(7) support, and passes eqn source
code through unchanged for now, which is more useful than what groff did.
However, as suggested by espie@, use the groff port when building Ports.
That always works because all ports using imake still have USE_GROFF=Yes.
thanks for useful hints to todd@ matthieu@ espie@ naddy@
tested by todd@ in Xenocara builds on i386, amd64, and macppc
ok matthieu@ and naddy@, and miod@ also agrees with the general direction
