Okay, I did that, and it seems to be this:

(defconst M2-mode-font-lock-keywords ' (
"///\\(/?/?[^/]\\|\\(//\\)*////[^/]\\)*\\(//\\)*///" .
'font-lock-string-face) )
(define-derived-mode M2-mode prog-mode "Macaulay2" (setq font-lock-defaults
'( M2-mode-font-lock-keywords )) )
(add-to-list 'auto-mode-alist '("\\.m2\\'" . M2-mode))