Require libmemcached

We are currently building moxi with libmemcached in our
product, and this commit is part of the process to simplify
our build process. By removing everything we don't use it'll
be easier to know which parts needs moving over to CMake.

Change-Id: I9f2073636099ba59bb645b6b867973a78289e958
Reviewed-on: http://review.couchbase.org/26952
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>