Target attributes if/unless

[Stefan Bodewig <bo...@bost.de>]

To me it doesn't feel right to have an if-attribute on targets but no
unless attribute - sometimes you can check for the absence of
something much better than for the presence.

One can argue that the include/exclude mechanisms of MatchingTask
provide everything you need - but I doubt so.

Any objections against adding an unless attribute to target?

Stefan