Be more verbose about changed environment.

If we have to re-resolve because an environment variable changed, print
its name as well as the old and new value.

Change-Id: I14032da14c151ec31a85f68f1ad56c3b44b3a9d4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>