* 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.