bootstrap: honor --no-git

bootstrap: honor --no-git

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

The intent behind the original addition of --no-git in commit
824f8d16 was to allow builds on a machine that either has no
git, or where git is prevented from using the network; yet where
the user had other means of checking out the correct version of
gnulib (such as using an NFS share).  But in practice, we were
still trying to use git, in spite of being told where gnulib-tool
already lives.  This fixes bootstrap to match the original
intent, by avoiding all direct use of git or submodules (any
indirect use of git, such as by autopoint, is beyond our control;
but at least that use does not require network access).  It is
still up to the end user to ensure that GNULIB_SRCDIR points
to the intended gnulib commit.

* build-aux/bootstrap: Don't even try to use git when user is
pointing to a static checkout.

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