Don't write directly to a make target ($@).

Don't write directly to a make target ($@).

Otherwise, if make is suspended, or killed with prejudice, or if the
system crashes, you could be left with an up-to-date, yet corrupt,
generated file.

I left off the `clean' addition, because I believe "make clean" should
not remove wildcard patterns like "*+", on the off-chance that someone
uses names like that for files they care about.  Besides, in practice,
those temporary files are left behind so rarely that they're not a bother,
and they're removed again as part of the next build.

[jc: sign-off?]

Signed-off-by: Junio C Hamano <junkio@cox.net>