Elegant way to hack port source

[Alejandro Imass]

Hi,

I need to modify a file from a port before building. Specifically, the
sane-backends pnm.c driver has a bug and the folks at the original
project has not fixed for a while. I need to modify pnm.c in the work
directory before compiling. What is an elegant way of doing this? If I
make and then modify, the main make file does not see the change made
in the file and will not recompile. And since there is no actual
makefile in the work subdirectory I can't compile there either!

There must be a FreeBSD way of dealing with modifying a port source
before compiling. Please advise.

Thanks in advance,
Alejandro Imass