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>

docs/compiling.html.in

@@ -65,8 +65,36 @@
       checkout it is necessary to generate the configure script and Makefile.in
       templates using the <code>autogen.sh</code> command. By default when
       the <code>configure</code> script is run from within a GIT checkout, it
-      will turn on -Werror for builds. This can be disabled with --disable-werror,
-      but this is not recommended. To build &amp; install libvirt to your home
+      will turn on -Werror for builds. This can be disabled with
+      --disable-werror, but this is not recommended.
+    </p>
+    <p>
+      Libvirt takes advantage of
+      the <a href="http://www.gnu.org/software/gnulib/">gnulib</a>
+      project to provide portability to a number of platforms.  This
+      is normally done dynamically via a git submodule in
+      the <code>.gnulib</code> subdirectory, which is auto-updated as
+      needed when you do incremental builds.  Setting the environment
+      variable <code>GNULIB_SRCDIR</code> to a local directory
+      containing a git checkout of gnulib will let you reduce local
+      disk space requirements and network download time, regardless of
+      which actual commit you have in that reference directory.
+    </p>
+    <p>
+      However, if you are developing on a platform where git is not
+      available, or are behind a firewall that does not allow for git
+      to easily obtain the gnulib submodule, it is possible to instead
+      use a static mode of operation where you are then responsible
+      for updating the git submodule yourself.  In this mode, you must
+      track the exact gnulib commit needed by libvirt (usually not the
+      latest gnulib.git) via alternative means, such as a shared NFS
+      drive or manual download, and run this any time libvirt.git
+      updates the commit stored in the .gnulib submodule:</p>
+    <pre>
+      $ GNULIB_SRCDIR=/path/to/gnulib ./autogen.sh --no-git
+    </pre>
+
+    <p>To build &amp; install libvirt to your home
       directory the following commands can be run:
     </p>
 

docs/hacking.html.in

@@ -35,7 +35,9 @@
   git checkout -t origin -b workbranch
   Hack, committing any changes along the way
 </pre>
-        <p>Then, when you want to post your patches:</p>
+        <p>More hints on compiling can be
+        found <a href="compiling.html">here</a>.  When you want to
+        post your patches:</p>
 <pre>
   git pull --rebase
   (fix any conflicts)