vips C and C++ programs can link directly against things in gobject
without appearing to, for example:
a = VIPS_OBJECT( b );
the macro will expand to include a g_type_check_instance_is_a(). So we
have to have gobject-2.0 in the link line of things like:
g++ -g -Wall hello8.cc `pkg-config vips-cpp --cflags --libs`
see also
https://github.com/jcupitt/libvips/issues/196