build: Update config.{guess,sub} to {2018-08-29,2018-08-29}.

* build-aux/config.guess: Update.
* build-aux/config.sub: Update.

Local changes of timestamp may result merge conflict when we update
those files from upstream. It's good to merge now than later.

• Fixes-commit: e99480affa595702595e59333aca96ab28389d34
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>