Oops, missed the commit message the last time around. No need to use
+= for variables defined only once.

Also, MAKE_FLAGS is defined with ?=, not += (probably a bad idea,
but hard to change now) , in bsd.port.mk, so you need the "-f" in there.
The only reason why this worked was because there is a "Makefile"
target in the source's Makefile.