PR 34922: fix ./configure --help=recursive

[Paolo Bonzini]

Ralf Wildenhues wrote:
> This makes
>   ./configure --help=recursive
> 
> work in the toplevel directory.  It is a hack as it uses undocumented
> Autoconf variables (ac_subdirs_all) and undocumened diversions
> (PARSE_ARGS).  It works with Autoconf 2.59 and current mainline.
> 
> Note that, due to a bug in Autoconf 2.59, it will not work outside of
> the source tree or an unconfigured tree; i.e.,
>   ../gcc/configure --help=recursive
> 
> only works after a prior configure run.  This bug has been fixed in
> Autoconf mainline.
> 
> I would need somebody to review and apply it for me (to both GCC and
> src).

I'll do it too.

Paolo