Fix occasional parallel make failures.

ep_engine.h includes generated_configuration.h (via
configuration.h). So every target that builds something that uses
ep_engine.h must also depend on ${CONFIG_SOURCE}.

Change-Id: Id5299324ae4c3fc764c10b0785b33ac132dad71b
Reviewed-on: http://review.couchbase.org/37974
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>