V3 PATCH: Export std::basic_iostream constructors/destructors

[Benjamin Kosnik]

Hey Mark, this patch looks fine. 

We're trying to keep 3.4/4.0 in sync WRT exports, so you'll have to
apply this to 3_4-branch as well, thanks.

> As these functions are part of an "extern template" they
> *must* be exported by the shared library, or we can indeed get link
> failure later.  It looks like someone has run into this problem
> before, in that there are entries for other constructors and
> destructors in the version map.

Yes, this is correct.

-benjamin