libstdc++, disable streams

[Benjamin Kosnik]

> It'll be nice to allow one to disable streams while using all other
> c++ library stuff - it can significantly decrease executable size.
> STLport does allow that.

I am unconvinced. I spent a lot of time tuning for (shared, not static)
size. Static binary size is another question altogether, and I think the
proper place for work on this is not in the library itself, but more
intelligence in the linker.

-benjamin