RFC: how to handle OS-specific configure options?

Hi all,
   as part of the configure-refactor work I'm currently reworking how
the various firewalling frameworks are handled.
In general, since the autoconf framework now correctly handles
tristate (yes/no/auto) options, I'm inclined to adopt where sensible
this general guideline: default should be "auto"
(enable-if-available), and fail the configuration if the user
force-enables something that can't be delivered.

Two consequences: squid will build more stuff in (interception is for
instance generally not included), and build-tests that force-enable
will fail because firewalling frameworks are mutually exclusive, so
more build tests will have to rely on the default "auto" approach.

I'd like to collect feedback on this approach. Reverting those options
to "do not build by default" is of course doable, even trivial.

Thanks

-- 
    /kinkie