GtkComboBoxEntry is deprecated in is GTK 2.23.
'Overview of Changes from GTK+ 2.22.0 to 2.23.0'
Thanks ... They're quite aggressive with the deprecations, it seems.
Not sure how to resolve this, if the suggested fix is switching to a widget (GtkComboBoxText) that doesn't exist at all in previous versions of GTK+.
As of 0.19.10, gentoo cleanly requires GTK+ 2.24.0 or greater, so this won't be fixed.