[v3] C99 complex math vs. std::complex

[Benjamin Kosnik]

> Thanks for the working on this: should fix regressions on many targets!

Here's hoping!

> While you are at it, before I forget: a few days ago I noticed that a 
> few 26_numerics/cmath testcases are protected by _GLIBCXX_USE_C99 (and I 
> added one ;) Should be changed, more correctly, to just 
> _GLIBCXX_USE_C99_MATH?

Maybe.

I don't know, off hand. I don't know what we should be attempting with
_GLIBCXX_USE_C99 at the moment, and without context. Can you elaborate?

best,
-benjamin