All these past years, I could always build Qt <= 5.5 from the Tarball (adding gitignore to be able to generate configure.exe)

But for 5.6 RC, after many attempts, I still can't build fully. More precisely, QtBase builds fine, but not the other modules. I am not speaking about compiler errors, but about include file problems.

I tried a prefix build, without more success. Even the exact documented way http://doc.qt.io/qt-5/windows-building.html fails.
Apart the tarball commercial package, I tried the open-source package, and the ZIP package, without success.

QTBUG-35314 or QTBUG-51324 were of no help.

And finally, to make sure, I have rebuilt 5.5.1 without any problem on the same machine...

So please just try this:
get the (commercial) tarball qt-everywhere-enterprise-src-5.6.0-rc.tar
and try building on Windows (following some standard procedure eg. http://doc.qt.io/qt-5/windows-building.html).