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

Commit 71d12562 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.

autogen.sh

@@ -72,8 +72,9 @@ if test -d .git; then
             echo "set env-var CLEAN_SUBMODULE to discard gnulib changes" 2>&1
             exit 1 ;;
     esac
+    # Keep this test in sync with cfg.mk:_update_required
     if test "$t" = "$(cat $curr_status 2>/dev/null)" \
-        && test -f "po/Makevars"; then
+        && test -f "po/Makevars" && test -f AUTHORS; then
         # good, it's up to date, all we need is autoreconf
         autoreconf -if
     else

cfg.mk

@@ -691,6 +691,8 @@ ifeq (0,$(MAKELEVEL))
   #  b653eda3ac4864de205419d9f41eec267cb89eeb .gnulib (v0.0-2286-gb653eda)
   # $ cat .git-module-status
   # b653eda3ac4864de205419d9f41eec267cb89eeb
+  #
+  # Keep this logic in sync with autogen.sh.
   _submodule_hash = sed 's/^[ +-]//;s/ .*//'
   _update_required := $(shell						\
       cd '$(srcdir)';							\