[Patch] Fix ac_c99_complex configury

[Paolo Carlini]

Benjamin Kosnik wrote:

>Hey, I think we can just do this, huh?
>
If it works, great! Yesterday, I did some experiments outside the
configury, "by hand" and was under the impression that indeed (as you
explained in the current comment) the backward/complex.h was picked,
then everything goes nuts ;)

Ah, now I see! The point is, when the C++ compiler is used by
*configure*, include/backward is *not* in the path!!! Do you say, in the
US: "Columbus egg?" ;)

Paolo.