RFC: Remove deprecated Makefile build

[elliottslaughter]

The Makefile build has been deprecated for over two years. In that time, we've had a few maintenance PRs, but not much (as far as I can tell) in the way of actual usage. Meanwhile, if we follow through with the LLVM 10 deprecation, we will completely lose our ability to test the Makefile on macOS. And of course, the CMake build has been more fully-featured basically since its inception.

Therefore, it seems to me that the time has come to finally remove the Makefile.

If you still use the Makefile (not CMake) build, please reply to this issue ASAP. As per our usual standard with RFCs, I'll leave the issue open for a while to collect replies, but if I don't hear anything I will probably remove the Makefile with the LLVM 10 code removal (or shortly after).

[elliottslaughter]

I'm hearing crickets on this one. Just another reminder that the Makefile will go away if no one speaks up, and CMake will be our only build system.

[elliottslaughter]

Final removal was performed in #637.