Makefile refactoring; NFC (#186)

* Use ?= for variables intended to be overridable.

* Use `override` consistently for variables not meant to be overridden.

* Omit miscellaneous comments from the build output.

* Tidy up some comments.

* Use `addprefix` to factor out a common prefix.

* Add a comment.

* Reorganize.

* Adjust indentation.

* Simplify the logic for the `check` rule.

* Remove the `override` keywords.

They theoretically protect what the Makefile considers to be
implementation details from being overridden on the command-line,
but in practice this isn't super important, and they add a lot of
clutter.

* Put source file names on their own lines.

Author: sunfishcode