Use system_header pragma in libstdc++-v3 headers only where required

[Phil Edwards]

I wrote:
> > (note 2.95.2 and other variants that don't support this pragma will have 
> > to disable -Werror now.)
> 
> 2.95.2 should already be doing so, via GLIBCPP_CHECK_COMPILER_VERSION.
> Dunno about other variants.


Hmmm.  The '-Werror'-disabling autoconf checks have bitrotted.  Current CVS
fails to build because of this exact problem.  Am fixing now.

Also, the pragma's actual name is missing from the message text:

    In file included from ../bits/std_limits.h:46,
                     from limitsMEMBERS.cc:37:
    [srcdir]/bits/std_cfloat.h:38: warning: ignoring pragma: 
    gmake[3]: *** [limitsMEMBERS.lo] Error 1


(Yes, it is system_header on line 38, I checked.  :-)

Phil