Enable superenv on Snow Leopard

I think we're ready to try enabling superenv for Snow Leopard users. When I still had a Snow Leopard machine, I tested a few dozen formulae and didn't find any specific superenv-related issues.

How much more testing do we want to do before enabling them? I could set the 10.6 test-bot to try building *everything*, or could just judge this good to go as is.