Fix accept_filter for GNU/Linux

[Poul-Henning Kamp]

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In message <20150902182340.793a7c35 at taghos.com.br>, Rafael Zalamena writes:

>Varnish cache daemon is not doing 'accept_filter' for the GNU/Linux
>version because of HAVE_ACCEPT_FILTER not being defined. Something along
>the way broke it, so in order to fix it I have added TCP_DEFER_ACCEPT
>detection code to configure.ac to define HAVE_ACCEPT_FILTER and
>HAVE_TCP_DEFER_ACCEPT so we can have a fallback of accept_filter on Linux.

For this to go in, it either needs to be off by default initially and
then we can change that to on by default once we have experience with it.


-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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Never attribute to malice what can adequately be explained by incompetence.