Use g_get_num_processors() when GLib >= 2.48.1
Use g_get_num_processors() when GLib >= 2.48.1

Also backport the patch from https://bugzilla.gnome.org/show_bug.cgi?id=748530

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