Rosegarden handles a missing qjackctl gracefully. It warns you about where to find this config item if qjackctl isn't found, will work fine if jackd is started by some other means, and will gracefully degrade its featureset if jack is missing.
Becuase of this, I'll be handling this dependency by adding a Suggests: dependency to qjackctl instead of a Depends: dependency. Thanks for the report. stew
