prog-mode not available in earlier Emacsen, need help with cfengine.el

[Ted Zlatanov]

I'd like, instead of saying 

(define-derived-mode cfengine3-mode prog-mode "CFEngine3" ...

to say something like:

(define-derived-mode cfengine3-mode (if (fboundp 'prog-mode) prog-mode 
fundamental-mode) "CFEngine3" ...

or something like it, which will keep the compiler happy and will work
in older Emacsen.  I'd rather not maintain two versions of cfengine.el.
I don't use any prog-mode features in cfengine.el.

Is that possible and OK with the maintainers, and if so, can I have an
example which works in Emacs 23.x and 24.x alike (and, I hope, XEmacs)?

Thanks
Ted