warning from make(1) about unclosed variable

[Roland Illig <roland.illig%gmx.de@localhost>]

Hi,

up to now, make(1) has not complained about the following line:

all:
	@echo ${UNCLOSED

I'd like to change that. The patch for this is pretty trivial, therefore
I wonder why nobody else has done this before.

A few test cases are already in usr.bin/make/unit-tests/varmisc.mk. Any
objections to add a warning for these cases? Later, when it proves to be
useful, it could even be upgraded to a parse error.

Roland