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.