Add more fine-grained rules on the Makefile.
Add more fine-grained rules on the Makefile.

This allows to compile each variant of `ncollide` individually.

steveklabnik/ncollide,sebcrozet/ncollide,MichaelRiss/ncollide,steveklabnik/ncollide,MichaelRiss/ncollide,sebcrozet/ncollide