Introduce a new option BOOTSTRAP that allows for doing a full
bootstrap of GCC, as opposed to just running a simple build. Bootstrapping is actually the default upstream, we disable it by default for the stable flavors of GCC since that is a huge win in terms of build time of the port. No change in default behavior for this port. (When bootstrapping, use bootstrap-lean instead of default/vanilla bootstrap to save on disk space.) Remove redundant setting of DISTNAME.