Make 'make' quieter while building git

Make 'make' quieter while building git

I find it difficult to see compiler warnings amongst the massive
spewing produced by GNU make as it works through our productions.
This is especially true if CFLAGS winds up being rather long, due
to a large number of -W options being enabled and due to a number
of -D options being configured/required by my platform.

By defining QUIET_MAKE (e.g. make QUIET_MAKE=YesPlease) during
compilation users will get a less verbose output, such as:

    ...
    CC builtin-grep.c
builtin-grep.c:187: warning: 'external_grep' defined but not used
    CC builtin-init-db.c
    CC builtin-log.c
    CC builtin-ls-files.c
    CC builtin-ls-tree.c
    ...

The verbose (normal make) output is still the default.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>