namespace issues with old C headers

[Benjamin Kosnik]

>I disagree; the standards committees are not forcing libstdc++ to
>promiscuously include headers, they are only PERMITTING it to do so.
>The standard is a minimum requirement only.

As stated before, any attempt to reduce extra inclusion is welcome.
Patches accepted! This is a no-brainer, and an aside to the other issue,
which is C99 vs C++ math.

-benjamin