[v3] install backwards headers in prefix

[Benjamin Kosnik]

I'm completely opposed to special-casing iostream.h. 

Furthermore, folks, we already have a macro for this stuff, see c++config.h:

// To enable older, ARM-style iostreams and other anachronisms use this.
//#define _GLIBCPP_DEPRECATED 1

Anyway. I believe the consensus (from Alexandre, Jason and I, at least) 
is to install backward headers in the normal place, and warn when they 
are used unless -Wno-deprecated is used. 

> 2. Add a compiler option ``-iobsolete'' which adds backward/ to the 
>    standard search path, so this file wouldn't be seen.

This remains as the only other sane option, seemingly. I'm uncertain if 
the compiler folks will agree with it though...

-benjamin