MB-10776: build NS_SERVER_VERSION based on PRODUCT_VERSION if available

Change-Id: I9069a043d249fa97485cbaeb99c4ad351baf8ec5
Reviewed-on: http://review.couchbase.org/35421
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Chris Hillery <ceej@couchbase.com>