Apparently I haven't tried a GTK2 build since the most recent set of
GTK-related code reorganisation. Some functions that were ifdef'ed out
in GTK3 builds were now unused even in GTK2 builds (and, because they
were also declared static, caused a -Werror build failure); and the
pointless stub version of gtkapp.c was missing a stub version of a
recently added function referred to from another module.