configure: fix issues that 'autoupdate' detected
configure: fix issues that 'autoupdate' detected

- AC_HELP_STRING is an obsolete macro and has been replaced with
  AS_HELP_STRING, so use it instead.

- AC_OUTPUT accepts arguments, but this use is deprecated. Instead, use
  AC_CONFIG_FILES and run AC_OUTPUT with no args.

sofar/buxton,sofar/buxton