BUILD: makefile: get rid of the config CFLAGS variable

BUILD: makefile: get rid of the config CFLAGS variable

CFLAGS currently is a concatenation of 4 other variables, some of which
are dynamically determined. This has long been totally unusable to pass
any extra option. Let's just get rid of it and pass the 4 variables at
the 3 only places CFLAGS was used. This will later allow us to make
CFLAGS something really usable.

This also has the benefit of implicitly restoring the build on AIX5
which needs to disable DEBUG_CFLAGS to solve symbol issues when built
with -g. Indeed, that one got ignored since the targets moved past the
CFLAGS definition which collects DEBUG_CFLAGS.