Fix typo in Makefile for install_native_objects

I was editing the makefiles while trying to pack Ctypes modules to resolve #100, when I noticed what looked like a typo. This typo is innocuous because it fails to set ```ctypes-foreign-unthreaded.install_native_objects``` to 'no', while the test later on checks whether it was set to 'yes', so we end up with the desired behaviour anyway.