build: submodule machinery now works also when no tag is reachable

The code in cfg.mk to detect when the git submodule was out of date
worked most of the time, but not when checked out in a certain way.
* cfg.mk: Extract submodule hash from command output and file,
and compare only that, since the format of the full line may vary.
Reported by Mike Burns, with some diagnosis by Daniel P Berrange.

cfg.mk

@@ -242,9 +242,17 @@ prev_version_file = /dev/null
 
 ifeq (0,$(MAKELEVEL))
   _curr_status = .git-module-status
-  _update_required :=							\
-    $(shell t=$$(git submodule status);					\
-      test "$$t" = "$$(cat $(_curr_status) 2>/dev/null)"; echo $$?)
+  # The sed filter accommodates those who check out on a commit from which
+  # no tag is reachable.  In that case, git submodule status prints a "-"
+  # in column 1 and does not print a "git describe"-style string after the
+  # submodule name.  Contrast these:
+  # -b653eda3ac4864de205419d9f41eec267cb89eeb .gnulib
+  #  b653eda3ac4864de205419d9f41eec267cb89eeb .gnulib (v0.0-2286-gb653eda)
+  _submodule_hash = sed 's/.//;s/ .*//'
+  _update_required := $(shell						\
+      actual=$$(git submodule status | $(_submodule_hash));		\
+      stamp="$$($(_submodule_hash) $(_curr_status) 2>/dev/null)";	\
+      test "$$stamp" = "$$actual"; echo $$?)
   ifeq (1,$(_update_required))
     $(error gnulib update required; run ./autogen.sh first)
   endif