Clean up config.m4

Change-Id: I970cad5f8747f820a7cae4966c962718ba52c2c9
Reviewed-on: http://review.couchbase.org/25659
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>