[QBS] Rule and the "usings" property

[Stéphane Fabry]

Hi,

thanks, I solved the problem in two steps :
- first rule as you said mostly does nothing but create an artifact the 
second rule can catch
- second multiplex rule does the job by catching target artifacts of the 
first one

Glad to see a change will improve this in the future !

And congrats for qbs, I'm just trying to switch from qmake, and now 
mostly done with a lot more flexibility, readability, functionnality and 
performance that the qmake solution i have.

Keep up the good work.

Regards !

Stéphane