MB-7810: Add -Wunused-variable to compiler warnings list

Change-Id: I3352562a066581dd336f20e3a5b49f847eea60d4
Reviewed-on: http://review.couchbase.org/24797
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>