custom build

[David Cake]

	How do I use MacPorts to first download source ready to 
install, then make some custom changes, then build?
	My actual aim is to add a particular type of decoder to 
wireshark that doesn't appear to currently be included (though code 
exists), which I am reasonable sure requires building from source. So 
the code changes I want to make are unlikely to interact with 
anything else in the system.
	Cheers
		David