Simplify CFLAGS/DEFINES in Makefile

Simplify CFLAGS/DEFINES in Makefile

I think the original intention was to make CFLAGS overridable
from the make command line, but somehow we ended up accumulating
conditional makefile sections that wrongly appends values to
CFLAGs.  These assignments do not work when the user actually
override them from the make command line!

DEFINES are handled the same way; it was seemingly overridable,
but the makefile sections had assignments, which meant
overriding it from the command line broke things.

This simplifies things by limiting the internal futzing to
ALL_CFLAGS, and by removing DEFINES altogether.  Overriding
CFLAGS from the command line should start working with this
change.

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