build: allow for local gnulib diffs

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.