Slightly edited these targets (release, debug, all) for consistency.
Also picked some nits in comments and 'make help'.

* Previously we were checking all CPP files, we don't need to check
test drivers (test*cpp) for style.
* Instead of *.h*, use a more correct *.h *.hpp

Changed compile flag from -std=c++17 to -std=c++1z for Mac users; still works on CAEN and autograder.