But nowhere do I find any of QT_NO_OPENGL or QT_OPENGL_ES* defined in the package (but then maybe I'm not looking in the right place? So I don't know why these two classes are included in the amd64 version of the package but not the armhf version.
The reason is that QT on armel/armhf is built for opengl ES rather than desktop opengl. I believe the reason for this is historic hardware support on armhf systems.