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).