Makefile: avoid timestamp updates to GIT-BUILD-OPTIONS

Makefile: avoid timestamp updates to GIT-BUILD-OPTIONS

We force the GIT-BUILD-OPTIONS recipe to run every time
"make" is invoked. We must do this to catch new options
which may have come from the command-line or environment.

However, we actually update the file's timestamp each time
the recipe is run, whether anything changed or not. As a
result, any files which depend on it (for example, all of
the perl scripts, which need to know whether NO_PERL was
set) will be re-built every time.

Let's do our usual trick of writing to a tempfile, then
doing a "cmp || mv" to update the file only when something
changed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>