Comment 5 for bug 1649530

This is strange. Lookup_icon is clearly defined in 3.16 as it was introduced (according to the documentation) in 2.4: https://developer.gnome.org/gtkmm/stable/classGtk_1_1IconTheme.html#a65eb6be589a121ece1bc5b267a033ab5.

Please check your gtkmm3 version, too and whether headers of it are properly installed.