Allow compilation with --enable-werror on macosx

Change-Id: I26d4e14627625524fc7b64cda911b5da205e0934
Reviewed-on: http://review.couchbase.org/28222
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>