Created attachment 4916 Remove-remained-USE_GTK_TOOLTIP_API.patch Seems like it was overlooked in the commit 1bd3d679d4591dd6689f0f7008e69d5fe8fe177e. Simple patch attached.
Mass-reassign all bugs from florian@ to goodies-dev@, thanks for the maintenance work! (and sorry for the bugmail spam..)
Sorry, totally overlooked your patch. Was actually fixed in http://git.xfce.org/panel-plugins/xfce4-datetime-plugin/commit/?id=5ba1a42b788424cfd5c845c47f538169ce190eda - thanks anyway!