Make configure.ac not so strict about autotools version

Change-Id: Ic8e247d4c951165ef379362e144b34eaf89e7895
Reviewed-on: http://review.couchbase.org/23843
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>