build: add target to run cppcheck

The advantage of having a target as opposed to running cppcheck by
hand

  - reuse list of source files
  - output errors in a format parsable, e.g. by emacs
  - returns exit code 1 on any error, for possibly use in other
    targets.

For the moment, leave this as an optional target. If desired, it can
be added to e.g. the release targets in the same way as the test
target.

Using two levels of directory for the stamps is arguably
overengineering, but it doesn't really cost anything, and leaves open
the possibility of putting other kinds of stamp files there.

This only checks "new" source files (w.r.t. their last check). A future target
(cppcheck-all ?) could blow away the stamp files first.

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