build: avoid infinite autogen loop

Several people have reported that if the .gnulib submodule is dirty,
then 'make' will go into an infinite loop attempting to rerun bootstrap,
because that never cleans up the dirty submodule.  By default, we
should halt and make the user investigate, but if the user doesn't
know why or care that the submodule is dirty, I also added the ability
to 'make CLEAN_SUBMODULE=1' to get things going again.

Also, while testing this, I noticed that when a submodule update was
needed, 'make' would first run autoreconf, then bootstrap (which
reruns autoreconf); adding a strategic dependency allows for less work.

* .gnulib: Update to latest, for maint.mk improvements.
* cfg.mk (_autogen): Also hook maint.mk, to run before autoreconf.
* autogen.sh (bootstrap): Refuse to run if gnulib is dirty, unless
user requests discarding gnulib changes.

.gnulib

@@ -1 +1 @@
-Subproject commit 440a1dbe523e37f206252cb034c3a62f26867e42
+Subproject commit b4938324b7113c9d73f8390d21f3ecbd842e74b9

autogen.sh

@@ -63,11 +63,22 @@ bootstrap_hash()
 if test -d .git; then
     curr_status=.git-module-status
     t=$(bootstrap_hash; git diff .gnulib)
+    case $t:${CLEAN_SUBMODULE+set} in
+        *:set) ;;
+        *-dirty*)
+            echo "error: gnulib submodule is dirty, please investigate" 2>&1
+            echo "set env-var CLEAN_SUBMODULE to discard gnulib changes" 2>&1
+            exit 1 ;;
+    esac
     if test "$t" = "$(cat $curr_status 2>/dev/null)" \
         && test -f "po/Makevars"; then
         # good, it's up to date, all we need is autoreconf
         autoreconf -if
     else
+        if test ${CLEAN_SUBMODULE+set}; then
+            echo cleaning up submodules...
+            git submodule foreach 'git clean -dfqx && git reset --hard'
+        fi
         echo running bootstrap$no_git...
         ./bootstrap$no_git --bootstrap-sync && bootstrap_hash > $curr_status \
             || { echo "Failed to bootstrap, please investigate."; exit 1; }

cfg.mk

@@ -700,10 +700,17 @@ ifeq (0,$(MAKELEVEL))
   _clean_requested = $(filter %clean,$(MAKECMDGOALS))
   ifeq (1,$(_update_required)$(_clean_requested))
     $(info INFO: gnulib update required; running ./autogen.sh first)
-Makefile: _autogen
+maint.mk Makefile: _autogen
   endif
 endif
 
+# It is necessary to call autogen any time gnulib changes.  Autogen
+# reruns configure, then we regenerate all Makefiles at once.
+.PHONY: _autogen
+_autogen:
+	$(srcdir)/autogen.sh
+	./config.status
+
 # Give credit where due:
 # Ensure that each commit author email address (possibly mapped via
 # git log's .mailmap) appears in our AUTHORS file.
@@ -718,13 +725,6 @@ sc_check_author_list:
 	  && echo '$(ME): committer(s) not listed in AUTHORS' >&2;	\
 	test $$fail = 0
 
-# It is necessary to call autogen any time gnulib changes.  Autogen
-# reruns configure, then we regenerate all Makefiles at once.
-.PHONY: _autogen
-_autogen:
-	$(srcdir)/autogen.sh
-	./config.status
-
 # regenerate HACKING as part of the syntax-check
 syntax-check: $(top_srcdir)/HACKING