Add PLATFORMS arg to DECLARE_DEP; Add platform-specific build dependencies

Change-Id: I26b88339da159394e71e4b2975593a043ab22d1a
Reviewed-on: http://review.couchbase.org/40290
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chris Hillery <ceej@couchbase.com>