I have a simple question - what is Shadow Building? I've seen it in Qt in Target Setup dialog, but don't know what it does or if I should use it.
It's also present in Projects tab, under General group.
Shadow building is a technique used to build different Qt builds of the same version for different platforms/compilers/etc. Your compiled build is in a different directory, separate from the original Qt source directory.
I've created a shadow build for my MSVS2010 compiler. If I wanted to, I could create a new build for MinGW in a different build directory.
See the definition here, and more information about configuring your build here.