Build system maintenance

* Makes ${QTXDG_LIBRARIES} also hold the dependencies libs
     ${QTXDG_LIBRARY} holds the qtxdg library itself. ${QTXDG_LIBRARIES}
     should hold qtxdg and all it's dependencies.
     See: http://www.cmake.org/Wiki/CMake:How_To_Find_Libraries

* Qt4: Replaces ${QT_LIBRARIES} by ${QT_module_LIBRARY}
* Qt5: Adds ${QTXDG_LIBRARY_DIRS} to the linker search paths