Hello Joachim,
I think the problem is that you need a capital d: -D:file=...
You can also try nant generateWebforms, which is quite fast anyways, and the number of forms is still small.
I think if you are not sure about an issue, it is easier to write in the forum, than to create an issue in the tracker. But it does not really matter, it is good both ways, thanks for reporting anything that you think is strange!
Thanks,
Timotheus