[External] : configure shell game

[Theo de Raadt]

> The warning message is somewhat useful: $() is POSIX now, and $() is
> much better than ``.

Unix has no way to emit a warning message without scripts observing it,
because warnings go to fd 2 stderr, which can and will be redirected
and parsed by something else.   As such, the warning message is not
useful because the only thing it does is break compatibliity.