build: automatically rerun ./bootstrap when needed

When "git pull" (or any other operation) brings in a new version of the
gnulib git submodule, you must rerun the autogen.sh script.  With this
change, "make" now fails and tells you to run ./autogen.sh, when needed.
* autogen.sh: Maintain a new file, .git-module-status, containing
the current submodule status.  If it doesn't exist or its content
is different from what "git submodule status" prints, then run
./bootstrap
* .gitignore: Add .git-module-status
* cfg.mk: Diagnose out of date submodule and fail.
* README-hacking: Update not to mention bootstrap.
* Makefile.am (MAINTAINERCLEANFILES): Add .git-module-status,
so that "make maintainerclean" will remove it.

cfg.mk

@@ -230,3 +230,13 @@ sc_libvirt_unmarked_diagnostics:
 
 # We don't use this feature of maint.mk.
 prev_version_file = /dev/null
+
+ifeq (0,$(MAKELEVEL))
+  _curr_status = .git-module-status
+  _update_required :=							\
+    $(shell t=$$(git submodule status);					\
+      test "$$t" = "$$(cat $(_curr_status) 2>/dev/null)"; echo $$?)
+  ifeq (1,$(_update_required))
+    $(error gnulib update required; run ./autogen.sh first)
+  endif
+endif