[racket-dev] SGC as default

I've changed the Racket CGC implementation --- which is mostly used
only to build the normal Racket variant --- to use SGC by default,
instead of the Boehm GC. The intent of the switch is to make the more
portable GC the default.

If you have an existing build in a repo checkout, then `make` is likely
to fail, because the makefile dependencies are not precise enough to
deal with the switch. You can discard your old build directory, or it
might work to simply delete

  <builddir>/racket/libmzgc.a

If you're using CGC and want to continue using the Boehm GC, then
provide `--disable-sgc` to `configure`. I've tuned SGC to bring its
performance closer to the Boehm GC, but it's still slower.