Qt's tagline is:
Write once, compile everywhere.
With this in mind, Qt doesn't officially offer any out of the box solution for cross compiling Qt applications from a specific platform to other different platforms. Though definitely do-able, if much work and time is invested, Qt good common practice would suggest you to build your Qt application directly on the targeted platform. That means, build your the Windows target for your application an a Windows machine, your Mac target on a Mac machine and Linux target on a, you guessed right( :-) ), a Linux machine.
That's way "Write once, compile everywhere", in my opinion is a very well chosen combination of words. Otherwise the tag line might have been, "Write once, compile for everything".
Going back to the original issue, you don't even need different physical machines for each of the targeted platforms, since in this day and age of good Virtualization solutions you can easily set up a couple of virtual machines and build you app on different platforms from the same physical machine.
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