The port should respect the CFLAGS variable. If it does not, please add NO_PACKAGE=ignores cflags to the Makefile.
An example of a Makefile respecting the CFLAGS variable follows. Note the +=:
CFLAGS += -Wall -Werror
Here is an example which does not respect the CFLAGS variable:
CFLAGS = -Wall -Werror
The CFLAGS variable is defined on FreeBSD systems in /etc/make.conf. The first example appends additional flags to the CFLAGS variable, preserving any system-wide definitions. The second example clobbers anything previously defined.