Add -Qunused-arguments for clang

Change-Id: I509250c16f6b65855fdcd20d5497f9adf1ad2509
Reviewed-on: http://review.couchbase.org/28677
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>