moved libmemcached under cmake

Change-Id: I47f8c6fbb59426f5e6e5e1f291c3400c01bdd0de
Reviewed-on: http://review.couchbase.org/30169
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>