LU-14432 configure.ac: fix AC_PREREQ

LU-14432 configure.ac: fix AC_PREREQ

AS_HELP_STRING was introduced in autoconf 2.58
requiring autoconf 2.69 is not necessary and it breaks
several builds in our Jenkins.

Change required autoconf to 2.59 just to be safe.

Change-Id: I30af873879866858d32b715ac4ce78d095e10f7d
Fixes: 8c9e2921 ("configure.ac: convert all help strings to use AS_HELP_STRING")
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/41665
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>