Introduce a new option BOOTSTRAP that allows switching between a
full bootstrap of GCC and just running a simple build.

Bootstrapping is the default upstream, and for very stable and old
flavors of GCC we disable this by default since that is a huge win
in terms of build time of the port. No change in default behavior
for this port -- for the time being we continue to bootstrap.

Update to the 20130822 snapshot of GCC 4.8.2.