I confirm what @jsulm, chromium mandates the use of the latest version of VisualStudio and explicitly does not support MinGW. You can find the information in the project page.
Sorry to raising this topic, but if somebody is also struggling with that, I finally found a way to build Qt5 (including libQt5WebEngine*.so) but not from .tar.xz provided but from Git, thanks to this page: https://wiki.qt.io/Building_Qt_5_from_Git