QMake compilation for the downloaded SoQt package
Updating the compilation settings for the downloaded package: before it was built with CMake, but that gave problems with an empty vtable for SoPlaneViewerP, which still has to be understood; now, I moved the compilation of the package to QMake, which appears to compile package fine.
Using QMake to compile the package does not add additional dependency, because SoQt needs Qt5 anyway.