qdbusxml2cpp cannot be used inside Qt

This is due to the fact that it links against QtCore and QtDBus and bootstrapping it like we do for moc/uic/rcc is not trivial.

We should probably link dbusxml2cpp without rpath and when calling it inside Qt automatically set LD_LIBRARY_PATH (or the platform equivalent) and when installing link the tool again with rpath against the intallation directory. The relinking is necessary since with some binutils/gcc versions rpath may override LD_LIBRARY_PATH, causing the tool to execute but possibly link against an already installed Qt version, which would be wrong and asking for trouble.