maint: improve usage of autogen's --no-git

maint: improve usage of autogen's --no-git

If you build libvirt with the --no-git option, then gnulib requires
either $GNULIB_SRCDIR in the environment or --gnulib-srcdir on the
command line.  But we had not been supporting its use from the
command line.  This patch is a bit picky: --gnulib-srcdir must be
passed immediately after --no-git; but since we don't reorder
arguments, and since we already required --no-git to be first to
have any effect, it's not too horrible.  It also does the wrong
thing if you include spaces in your arguments, but developers
doing that deserve what they get, right?

Signed-off-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>