Fix SHLIB_COMPAT with undefined obsoleting version

[Andreas Schwab]

Chris Metcalf <cmetcalf@tilera.com> writes:

> What was confusing is that we appear to build pthread_atfork.os and
> then we don't use it.

That's a bug, it shouldn't even be built.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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