How to override port Makefile to point to local directory?

I am writing software that I would eventually like to install on 
FreeBSD. I'm currently stuck trying to write a Makefile that will point 
to my local git dir, so I can build and install it while I'm developing. 
All of the finished Makefiles reference tar packages, which makes sense. 
But when I'm actively developing, I don't want to commit the code, push 
it, have it built, update the makefile with the commit, and make.

Does anyone know of a way to override options to a port's Makefile to 
point to a local directory on disk? I want to run "sudo make install" 
and "make package" from my development directory, using the same 
Makefile that would be in the ports tree.

Pat