LU-16240 build: Use new AS_HELP_STRING

LU-16240 build: Use new AS_HELP_STRING

Starting with autoconf 2.70 AC_HELP_STRING has been replaced with
AS_HELP_STRING. Move to this new macro.

Test-Parameters: trivial
Change-Id: I1d4f69fb844f51f05a8f46751df8b79d93db78f8
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48884
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>