Move libmemcached to cmake build

Change-Id: I55a217daa5dfb32b94353dfee8ed48ea6de0fd60
Reviewed-on: http://review.couchbase.org/30171
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>