Latest snapshot won't build with --enable-libstdcxx-v3

[Joe Buck]

> bkoz@redhat.com said:
> >> messages. Why having done that anyway ??? I fill somewhat responsible 
> >> because this redirection was not here before I complained about the 
> >> use of Werror.
> > it was always there  
> 
> But what is the rationale behind it ???

I wasn't the person who decided to put in -Werror, but I think it is
a very good idea for libstdc++ to use it.

Warnings produced by the inclusion of system library header files are not
acceptable, because it means that no user of the library can use flags
like -Wall without being distracted by warnings from the system library.
(Old-time Cygnoids are sure to remember how I always used to beat them
up for this - libg++ used to always generate piles of warnings, I'd send
lots of little patches to fix them, and then the next release would
put them all back again).

Putting in -Werror forces the developers to produce warning-free code.

(Now, warnings in files that will never be compiled by library users
aren't a problem, but we currently don't have a way to change the
warning level every time we cross an #include boundary).