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* cfg.mk: Diagnose out of date submodule and fail.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.