local patch to a /usr/ports file? how to do it?

[Aaron Wohl]

I have a patch to /usr/ports/misc/amanda-clients.  Its a one line change
to sendbackup-dump.c to add the L switch to snapshot for 5.x when making
a dump.  Where do I put the patch so when I do make in
/usr/ports/misc/amanda-clients it gets applied?  I checked the handbook
and porters guide... I dont see how to have a local change.

The patch im using isnt that great to submit... it just forces L on dump.
 It doesnt let amanda control when to use L or not.  Im just using this
till something more real comes along.