Make CFLAGS overridable from make command line.

Make CFLAGS overridable from make command line.

This patch renames COPTS to CFLAGS, because it's COPTS that was user
overridable.  Also, -Wall is moved there because it's optional.  What
was CFLAGS is now ALL_CFLAGS, which users should not override.

Defines are added to DEFINES.  Since ALL_CFLAGS is recursively expanded,
it uses the final value of DEFINES.

Implicit rules are made explicit since the implicit rules use CFLAGS
rather than ALL_CFLAGS.  I believe that serious projects should not rely
on implicit rules anyway.  Percent rules are used because they are used
already and because they don't need the .SUFFIXES target.

[jc: in addition to updating the patch for 0.99.4, I fixed up a
glitch in Pavel's original patch which compiled sha1.o out of
mozilla-sha1/sha1.c, where it should have left the resulting
object file in mozilla-sha1 directory for later "ar".]

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>