The GTK dependency listed in configure.ac is incorrect. It lists 2.4.0 as the minimum version of GTK. The actual minimum version of GTK is 2.6 or newer.
TilEm uses gtk_combo_box_get_active_text(), which is only available in 2.6 onward. See the link below.
Thank you for the report :)
A new version of TilEm was released since this old message.
We could forget about it :)