[libstdc++] Separate distance(i,j,n) extension from internal routines.

[Paolo Carlini]

Phil Edwards wrote:

> This has been tossed around on the libstdc++ list lately.  This particular
> patch was tested on i686-linux; the general idea met with approval from
> everybody.  Paolo is waiting on this one so he can continue with other
> things, so I'm checking it in before I leave for the day.

Thanks Phil!
I have to questions inspired by your patch:
1- What about backward/iterator.h ? Should we modify it as I have done for backward/algo.h
?
2-

> +#pragma GCC system_header

Could you possibly explain a bit the meaning of this #pragma? I did'nt put it in my
ext/algorithm? Should I correct it?

Thanks,
Paolo.