Add Makefile that invokes cargo-c

[cpu]

We switched from GNU Make to cargo-c for Windows/Apple builds due to the complexity involved in maintaining the old Makefile and its build/install logic (especially with the added complexity of new features, mutually exclusive features, different build modes, pkg-config etc).

However, it would be nice if we left a Makefile that had the expected build/install targets that itself just deferred to running cargo-c. That adds little to no maintenance burden but presents a familiar interface to folks used to make. It will also likely ease the transition for downstream projects that previously relied on our makefile.