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* cfg.mk: Diagnose out of date submodule and fail.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

1 files changed, 11 insertions, 0 deletions

diff --git a/autogen.sh b/autogen.sh
index e6bde33ae..415f3ec6d 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -54,6 +54,17 @@ if test -z "$*"; then
         echo "to pass any to it, please specify them on the $0 command line."
 fi
 
+# Ensure that whenever we pull in a gnulib update or otherwise change to a
+# different version (i.e., when switching branches), we also rerun ./bootstrap.
+curr_status=.git-module-status
+t=$(git submodule status)
+if test "$t" = "$(cat $curr_status 2>/dev/null)"; then
+    : # good, it's up to date
+else
+  echo running bootstrap...
+  ./bootstrap && echo "$t" > $curr_status
+fi
+
 # Automake requires that ChangeLog exist.
 touch ChangeLog