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

[Paolo Carlini]

Gabriel Dos Reis wrote:

>(tangentially, does anybody know that _BUGGY_COMPLEX is still
>relevant?  That is for so_7 branch...)
>  
>
Since May, 2001, nothing in the autoconf machinery sets it anymore
and seems completely dead.

If people agree, I can just remove it, together with a few small additional
tweaks and clean-ups that I mentioned yesterday.

Paolo.