RFR: 7200065 Cross-compilation changes to support the new-build

[David Holmes]

webrev:

http://cr.openjdk.java.net/~dholmes/7200065/webrev/

These are Fredrik's contributions from build-infra being moved to 
mainline (to join the changes Kelly pushed the other day). There is a 
slight change to Fredrik's original changeset due to a bug with the old 
build (PATH_SEP wouldn't get set) - and it's simpler to use ?:= than ifdef.

I'm acting as one reviewer but would welcome other eyes.

Thanks,
David