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

[Benjamin Kosnik]

In and working, thanks to both Zack and Loren!
(note 2.95.2 and other variants that don't support this pragma will have 
to disable -Werror now.)

-benjamin

> 2000-05-16  Loren J. Rittle  <ljrittle@acm.org>
> 
> 	* bits/std_cassert.h: Use system_header pragma.
> 	* bits/std_cctype.h: Likewise.
> 	* bits/std_cerrno.h: Likewise.
> 	* bits/std_cfloat.h: Likewise.
> 	* bits/std_climits.h: Likewise.
> 	* bits/std_clocale.h: Likewise.
> 	* bits/std_cmath.h: Likewise.
> 	* bits/std_csetjmp.h: Likewise.
> 	* bits/std_csignal.h: Likewise.
> 	* bits/std_cstdarg.h: Likewise.
> 	* bits/std_cstddef.h: Likewise.
> 	* bits/std_cstdio.h: Likewise.
> 	* bits/std_cstdlib.h: Likewise.
> 	* bits/std_cstring.h: Likewise.
> 	* bits/std_ctime.h: Likewise.
> 	* bits/std_cwchar.h: Likewise.
> 	* bits/std_cwctype.h: Likewise.
> 	* bits/std_exception.h: Likewise.
> 	* bits/std_new.h: Likewise.
> 	* bits/std_typeinfo.h: Likewise.
> 	* shadow/bits/std_cassert.h: Likewise.
> 	* shadow/bits/std_cctype.h: Likewise.
> 	* shadow/bits/std_cerrno.h: Likewise.
> 	* shadow/bits/std_cfloat.h: Likewise.
> 	* shadow/bits/std_climits.h: Likewise.
> 	* shadow/bits/std_clocale.h: Likewise.
> 	* shadow/bits/std_cmath.h: Likewise.
> 	* shadow/bits/std_csetjmp.h: Likewise.
> 	* shadow/bits/std_csignal.h: Likewise.
> 	* shadow/bits/std_cstdarg.h: Likewise.
> 	* shadow/bits/std_cstddef.h: Likewise.
> 	* shadow/bits/std_cstdio.h: Likewise.
> 	* shadow/bits/std_cstdlib.h: Likewise.
> 	* shadow/bits/std_cstring.h: Likewise.
> 	* shadow/bits/std_ctime.h: Likewise.
> 	* shadow/bits/std_cwchar.h: Likewise.
> 	* shadow/bits/std_cwctype.h: Likewise.
> 	* shadow/bits/wrap_libio.h: Likewise.
> 	* shadow/bits/wrap_unistd.h: Likewise.
> 	* shadow/sys/cdefs.h: Likewise.