[PATCH] Properly check linker option in LIBC_LINKER_FEATURE [BZ #28738]

[Florian Weimer]

* H. J. Lu via Libc-alpha:

> Update LIBC_LINKER_FEATURE to also check linker warning message since
> unknown linker -z option may be ignored by linker:
>
> $ touch x.c
> $ gcc -shared -Wl,-z,foobar x.c
> /usr/bin/ld: warning: -z foobar ignored
> $ echo $?
> 0
> $
>
> This fixes BZ #28738.

Looks okay to me.

Thanks,
Florian