It would fall into the glib code path because it was only checking
if QT_NO_GLIB was defined, but ignoring the env variable.

gst_bus_add_watch_full only works with a glib event loop running.

Task-Id: QTBUG-51607
Change-Id: I726afd5d6e114eacea6e5bc71e7a6e2d1c5bbd74
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>