build: allow for local gnulib diffs

Commit f7bd00c12 pulled in a gnulib module that fails to compile
on mingw.  While it would be nice to pull in a newer version of
.gnulib that fixes this, it is difficult to backport any .gnulib
update to older releases.  So, it makes sense to take advantage
of gnulib-tool's ability to support local diffs, where we can
apply specific diffs in our use of gnulib without waiting for
upstream gnulib to pick up those changes, as well as avoiding
a wholesale .gnulib update.  The existence of local diffs will
also make it easier to backport fixes against a tarball (as long
as a tarball and libvirt.git share the same .gnulib commit, then
the tarball can be patched by applying the same local diffs as
a post-release libvirt.git commit, without having to rerun an
entire gnulib-tool bootstrap).

This patch introduces the framework for supporting local diffs,
without actually introducing any.

* bootstrap.conf (local_gl_dir): New variable.
* autogen.sh (bootstrap_hash): Hash any local diffs, to force a
re-bootstrap if just diffs change.
* cfg.mk (_update_required): Likewise.

autogen.sh

@@ -41,10 +41,13 @@ fi
 # is required.  The first is just the SHA1 that selects a gnulib snapshot.
 # The second ensures that whenever we change the set of gnulib modules used
 # by this package, we rerun bootstrap to pull in the matching set of files.
+# The third ensures that whenever we change the set of local gnulib diffs,
+# we rerun bootstrap to pull in those diffs.
 bootstrap_hash()
 {
     git submodule status | sed 's/^[ +-]//;s/ .*//'
     git hash-object bootstrap.conf
+    git ls-tree -d HEAD gnulib/local | awk '{print $3}'
 }
 
 # Ensure that whenever we pull in a gnulib update or otherwise change to a