bootstrap: use GNULIB_SRCDIR to reduce disk usage

bootstrap: use GNULIB_SRCDIR to reduce disk usage

This slightly changes the semantics of GNULIB_SRCDIR; previously,
it was a way to change the contents of .gitmodules to actually
point to a new location, which meant that running:
 GNULIB_SRCDIR=~/gnulib ./bootstrap
would result in a dirty tree.  The new semantics are nicer; the
submodule is cloned as normal, but the clone uses the original
$GNULIB_SRCDIR as an object reference to reduce download time
and disk space.

* build-aux/bootstrap (GNULIB_SRCDIR): If set, use as a reference,
rather than reconfiguring where the submodule points.

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