Require libconflate

We are using libconflate in our product so it shouldn't be
optional. This simplifies the move towards CMake

Change-Id: Icb49849018d8c0a60436c62a5a87114edf470eaf
Reviewed-on: http://review.couchbase.org/26953
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>