bootstrap: use correct source when copying build-aux files

bootstrap: use correct source when copying build-aux files

in my setup for ttfautohint (http://repo.or.cz/w/ttfautohint.git) I
have the following assignment in `bootstrap.conf':

  build_aux=gnulib

It seems that the patch below is necessary to make this really work
(at least it works for me): if I understand the issue correctly,
$build_aux is a *destination* variable, so it must not be used for the
files listed in $gnulib_extra_files which uses the directory structure
of the `gnulib' repository.

* build-aux/bootstrap (gnulib_extra_files): This variable is
relative to upstream gnulib layout, not downstream.

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