[Patch] Fix ac_c99_complex configury

[Benjamin Kosnik]

> Hmm, I'm confused.  But, yes, I agree we may probably stop 
> <complex.h> since it now means something different in C99.  
> Hmm.  C++ + C99 == mess.

I agree with the C++ vs. C99 sentiment.

However, it is not exactly clear what you mean WRT backwards/complex.h. 

I'd like to remove it. Since backwards is searched by default for C++
compiles, and there is no easy way to remove this from the include
search path,  it looks like this will be a serious issue WRT libstdc++
vs. libc.

Is this what you were implying? I think you are right WRT to the rest
of the deprecated headers, at least the ones in the C++ standard under
annex D. We have to keep these for the time being.

 -benjamin