reorganize Makefile to support "least surprise" config/make semantics

reorganize Makefile to support "least surprise" config/make semantics

the major change here is that CFLAGS is now a variable that can be
changed entirely under user control, without causing essential flags
to be lost. previously, "CFLAGS += ..." was valid in config.mak, but
using "CFLAGS = ..." in config.mak would have badly broken the build
process unless the user took care to copy the necessary flags out of
the main Makefile.

I have also added a distclean target that removes config.mak.