build: fix 'make check' with newer git

Newer git doesn't like the maint.mk rule 'public-submodule-commit'
run during 'make check', as inherited from our checkout of gnulib.
I tracked down that libvirt commit 8531301d picked up a gnulib fix
that makes git happy.  Rather than try and do a full .gnulib
submodule update to gnulib.git d18d1b802 (as used in that libvirt
commit), it was easier to just backport the fixed maint.mk from
gnulib on top of our existing submodule level.  I did it as follows,
where these steps will have to be repeated when cherry-picking this
commit to any other maintenance branch:

mkdir -p gnulib/local/top
cd .gnulib
git checkout d18d1b802 top/maint.mk
git diff HEAD > ../gnulib/local/top/maint.mk.diff
git reset --hard
cd ..
git add gnulib/local/top
Signed-off-by: NEric Blake <eblake@redhat.com>

gnulib/local/top/maint.mk.diff

+diff --git c/top/maint.mk w/top/maint.mk
+index bf8f53a..d5bb427 100644
+--- c/top/maint.mk
++++ w/top/maint.mk
+@@ -2,7 +2,7 @@
+ # This Makefile fragment tries to be general-purpose enough to be
+ # used by many projects via the gnulib maintainer-makefile module.
+
+-## Copyright (C) 2001-2013 Free Software Foundation, Inc.
++## Copyright (C) 2001-2014 Free Software Foundation, Inc.
+ ##
+ ## This program is free software: you can redistribute it and/or modify
+ ## it under the terms of the GNU General Public License as published by
+@@ -591,7 +591,7 @@ sc_prohibit_root_dev_ino_without_use:
+
+ sc_prohibit_openat_without_use:
+ 	@h='openat.h' \
+-	re='\<(openat_(permissive|needs_fchdir|(save|restore)_fail)|l?(stat|ch(own|mod))at|(euid)?accessat)\>' \
++	re='\<(openat_(permissive|needs_fchdir|(save|restore)_fail)|l?(stat|ch(own|mod))at|(euid)?accessat|(FCHMOD|FCHOWN|STAT)AT_INLINE)\>' \
+ 	  $(_sc_header_without_use)
+
+ # Prohibit the inclusion of c-ctype.h without an actual use.
+@@ -1283,15 +1283,17 @@ vc-diff-check:
+ rel-files = $(DIST_ARCHIVES)
+
+ gnulib_dir ?= $(srcdir)/gnulib
+-gnulib-version = $$(cd $(gnulib_dir) && git rev-parse --short HEAD)
++gnulib-version = $$(cd $(gnulib_dir)				\
++                    && { git describe || git rev-parse --short=10 HEAD; } )
+ bootstrap-tools ?= autoconf,automake,gnulib
+
++gpgv = $$(gpgv2 --version >/dev/null && echo gpgv2 || echo gpgv)
+ # If it's not already specified, derive the GPG key ID from
+ # the signed tag we've just applied to mark this release.
+ gpg_key_ID ?=								\
+   $$(cd $(srcdir)							\
+      && git cat-file tag v$(VERSION)					\
+-        | gpgv --status-fd 1 --keyring /dev/null - - 2>/dev/null	\
++        | $(gpgv) --status-fd 1 --keyring /dev/null - - 2>/dev/null	\
+         | awk '/^\[GNUPG:\] ERRSIG / {print $$3; exit}')
+
+ translation_project_ ?= coordinator@translationproject.org
+@@ -1393,8 +1395,8 @@ public-submodule-commit:
+ 		&& git --version >/dev/null 2>&1; then			\
+ 	  cd $(srcdir) &&						\
+ 	  git submodule --quiet foreach					\
+-	      test '"$$(git rev-parse "$$sha1")"'			\
+-	      = '"$$(git merge-base origin "$$sha1")"'			\
++	      'test "$$(git rev-parse "$$sha1")"			\
++		  = "$$(git merge-base origin "$$sha1")"'		\
+ 	    || { echo '$(ME): found non-public submodule commit' >&2;	\
+ 		 exit 1; };						\
+ 	else								\