patch to add make -U

[Faried Nawaz]

While working around a port issue (ports/55013), I discovered that
make couldn't unset variables using make -U.  I've written a small
patch that adds -U functionality, but I haven't tested it extensively.

http://web.nilpotent.org/tmp/make.diff.bz2  (~ 3KB unpacked)
against yesterday's -CURRENT code.

A simple Makefile I used to test it:

-- cut here --
FOO = bar

.ifdef FOO
SAY = y
.else
SAY = n
.endif

all:
	echo $(SAY)
-- cut here -- 

Try "make -U FOO".



Faried.
-- 
The Great GNU has arrived, infidels, behold his wrath !
"If a MOO runs on a port no one accesses, does it run?"