Ever since commit 7b21981c started generating AUTHORS, we now have
the situation that if you flip between two branches in the same
git repository that cross that commit boundary, then 'make' will
fail due to automake complaining about AUTHORS not existing.  The
simplest solution is to realize that if AUTHORS does not exist,
then we flipped branches so we will need to rerun bootstrap
anyways; and rerunning bootstrap ensures AUTHORS will exist in time.

* cfg.mk (_update_required): Also depend on AUTHORS.