#7 Split CFLAGS in makefile

[Ed Avis]

At present if you want to build arcem with different
optimizations you have to edit the makefile. You can't
pass in CFLAGS on the command line because you'll lose
all the -Dwhatever options needed to make it build.
This patch splits out the optimization flags from the
others, so you can say

% make CFLAGS=-O3

to build with those optimizer switches. This will be
useful for making packaged versions of arcem.

If no CFLAGS is specified on the make command line then
the default flags are the same as before.

[Ed Avis]

Patch to Makefile to let CFLAGS be for optimizer flags only