add example compile lines to C docs
add example compile lines to C docs

see https://github.com/jcupitt/libvips/issues/909

lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips