It appears that (at least in Qt5, not sure about Qt4) SVG icons are not always displayed per default. On Debian for example, installing just "qtbase5-dev" is not sufficient. After installing libqt5svg5-dev (probably only the lib itself is needed, not the -dev package, but this is untested) the icons appear and work fine. We might need to add the equivalent of qmake's "QT += svg" in the cmake config to force the usage of the Qt SVG code.
Fixed in 726ea66712afefafd96b4f08667fa97ab741c343.