updating mk/gnu-config

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

I recently found out that our autoconf package distributes old
versions of config.guess and config.sub. The reason is that pkgsrc
copies the file residing in mk/gnu-config/config.{sub,guess} into
every GNU_CONFIGURE-using packages working directory, under the
assumption that the pkgsrc-provided files are usually newer and better
than the ones coming with the package.

Obviously, this assumption is currently wrong at least for the
autoconf package :)

So I'll replace config.{guess,sub} and 'missing' with the latest
versions from autoconf-2.69 (after the branch).

This directory contains two more files: libtool-1.4.m4 and
config.rpath, according to their headers coming from libtool, I guess
around version 1.4.

libtool-2.4.2, the current pkgsrc version, doesn't provide
config.rpath, and has lots of m4 files. IMO, when you want to use
these in a package, you should BUILD_DEPEND on libtool-m4 instead. So
I think these two files should just be removed (as well as the related
code in configure/config-override.mk and
platform/{Cygwin,Interix}.mk). Am I overlooking anything here?

Cheers,
 Thomas