build: fix incremental autogen.sh when no AUTHORS is present

build: fix incremental autogen.sh when no AUTHORS is present

Commit 71d1256 tried to fix a problem where rebasing an old
branch on top of newer libvirt.git resulted in automake failing
because of a missing AUTHORS file.  However, while the fix
worked for an incremental 'make', it did not work for someone
that directly reran './autogen.sh'.  Reported by Laine Stump.

* autogen.sh (autoreconf): Check for same conditions as cfg.mk.
* cfg.mk (_update_required): Add comments.
(cherry picked from commit 55dc872bd85e0756a13fc24693c4eed31d0c202f)