Well looking at your config.log here is where it fails:
/home/netbsd/pkgsrc/x11/gtk2/work/.buildlink/lib/libcairo.so: undefined
reference to `pixman_image_set_destroy_function'
/home/netbsd/pkgsrc/x11/gtk2/work/.buildlink/lib/libcairo.so: undefined
reference to `pixman_image_composite32'
/home/netbsd/pkgsrc/x11/gtk2/work/.buildlink/lib/libcairo.so: undefined
reference to `pixman_image_fill_boxes'
libcairo was linked to libpixman, but for some reason libpixman's
pixman_image_* functions are not available.
Do you have two libpixman libraries installed, one under /usr/X11R7/lib and
one under /usr/pkg/lib? If so, then it finds the wrong version of library,
it's better to stick with one system-wide libpixman.
Maybe pkg_delete libpixman and rebuild libcairo.