We are starting work to remove the osgEarthQt nodekit from the repository. This follows in line with OSG removing core Qt support. The complexity of supporting now and future Qt versions is outside the scope of the osgEarth SDK.
For now, the plan is to attempt to put the ViewWidget class into a standalone osgearth_qt example app along with all Qt build support CMAKE code. If you use ViewWidget in your software feel free to simply copy/paste/adapt the code and use it directly in your application instead of linking to the osgEarthQt node kit.