Use toplevel CMakeLists.txt

Change-Id: I3b543fcb7673dcc4b8e3c3fcfd3338612359e3c6
Reviewed-on: http://review.couchbase.org/28727
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>