Update to the 20130518 snapshot of GCC 4.7.4.
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.