As of EQL5 version 17.2.4, I just successfully tested building with Qt 5.5.1, both on Linux on Windows.
On Windows, both MSVC and MinGW seem to work (although MinGW needs some manual adaption, see README-1.txt).
Why Qt 5.5.1?
Because it is the last one containing QtWebKit, and it is sufficient to run all examples.
Link to Qt 5.5.1 downloads:
https://download.qt.io/archive/qt/5.5/5.5.1/Paul