[BUILD] major rework of the GNU Makefile

[BUILD] major rework of the GNU Makefile

The build process was getting annoying under some conditions,
especially on platforms which are used to set CFLAGS, as well
as those which set a lot of complex defines. The new Makefile
takes care of this situation by not mixing TARGET, CPU and user
values, and by making privileging the pre-setting of common
variables with the ability to override them.

Now CFLAGS and LDFLAGS are set by default and may be overridden
without the risk of breaking useful defines. Options are better
dealt with, and as a bonus, it was possible to merge the FreeBSD
and OpenBSD targets into the common GNU Makefile.

The report of build options by "haproxy -vv" has been slightly
adapted to the new mode. Options implied by architecture are not
reported, only user-specified options are. It is also possible to
add options which will not be reported in order not to mangle the
output when specifying dirty informations such as URLs...

The Makefile was copiously documented and it should be easier to
build for any target now. Backwards compatibility with older
build processes was kept, and warnings are emitted for deprecated
build options.