gcc builtin functions, e.g. memcpy, and namespace std

[Benjamin Kosnik]

> 4. Treat ::memcpy and std::memcpy identical, as any extern "C"
>    function with a certain name should be the same function, regardless
>    in which namespace they are defined.

right. Note, that we need this for some C structs as well. It would be
great to have this. ...

Nathan wrote up a bit describing what was necessary, a ways ago. Maybe he
can provide a pointer....

-benjamin