PCBC-230: Support older autconf by not using AC_CHECK_HEADERS_ONCE.

Change-Id: Ib500f4f750dba93b5bc9c338afd1269f011459ae
Reviewed-on: http://review.couchbase.org/26210
Reviewed-by: Pierre Joye <pierre.php@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>