I see a couple -Wformat warnings when compiling gnome-initial-setup:

gis-privacy-page.c: In function 'activate_link':
gis-privacy-page.c:255:41: warning: not enough variable arguments to fit a sentinel [-Wformat=]
                                         NULL);
                                         ^

I think gtk_dialog_new_with_buttons should be called with two trailing NULLs when there are no buttons to add: the first NULL for the button text, and the second NULL to end the va_list. That avoids these warnings.