Best way to override user's CFLAGS in a port?

[John Polstra]

The ezm3 port has to be built with an optimization level of -O, and
that's not likely to change any time soon.  Some users, especially
users of -current where -O2 is officially supported, are getting
burned by that.  What is the best way for me to change the ezm3 port's
Makefile so that -O will be used regardless of the user's environment
or /etc/make.conf settings?

Thanks,
John