Build failed in OSX (TCP_DEFER_ACCEPT)

I tried to build, but rake aborted.

Logs:

     bossan_ext.c:1536:35: error: use of undeclared identifier 'TCP_DEFER_ACCEPT'
       r = setsockopt(fd, IPPROTO_TCP, TCP_DEFER_ACCEPT, &on, sizeof(on));

We can use TCP_DEFER_ACCEPT option only in Linux (2.4+).
This patch is quick-fix for not Linux Systems.