build: honor autogen.sh --no-git

build: honor autogen.sh --no-git

Based on a report by Chandrashekar Shastri, at
https://bugzilla.redhat.com/show_bug.cgi?id=979360

On systems where git cannot access the outside world, a developer
can instead arrange to get a copy of gnulib at the right commit
via side channels (such as NFS share drives), set GNULIB_SRCDIR,
then use ./autogen.sh --no-git.  In this setup, we will now
avoid direct use of git.  Of course, this means no automatic
gnulib updates when libvirt.git updates its submodule, but it
is expected that any developer in such a situation is already
prepared to deal with the fallout.

* .gnulib: Update to latest, for bootstrap.
* bootstrap: Synchronize from gnulib.
* autogen.sh (no_git): Avoid git when requested.
* cfg.mk (_update_required): Skip automatic rerun of bootstrap if
we can't use git.
* docs/compiling.html.in: Document this setup.
* docs/hacking.html.in: Mention this.
* HACKING: Regenerate.

Signed-off-by: Eric Blake <eblake@redhat.com>