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

[Benjamin Kosnik]

> Ok. I'll try to disentangle a bit this soon. Indeed, there is an XXX in 
> a comment...

Saw it. I agree: I like your proposed model, where

_GLIBCXX_USE_C99 means _GLIBCXX_USE_C99_MATH && _GLIBCXX_USE_C99_COMPLEX && _GLIBCXX_USE_C99_WCHAR.

When we need finer grained control, we can use the less strong macros.

Sounds good to me, and resolves the current ambiguity.

-benjamin