On Cygwin and NetBSD 7.0+ struct in_pktinfo has no ipi_spec_dst field, which
caused nginx compilation error.  Now presence of this field is ensured by the
IP_PKTINFO feature test.

The problem was introduced by dbb0c854e308 (1.13.0).

The sched_setaffinity() function was introduced in DragonFly BSD 4.7,
so it is no longer Linux-specific.

Prodded by Sepherosa Ziehau.

Oracle Developer Studio 12.5 introduced GCC-compatible __sync builtins.
Unfortunately, these builtins are neither GCC-compatible (they generate
warnings when used with volatile), nor working (unexpectedly fail on
unpredictable combinations of code layout and compiler flags).  As such,
the gcc builtin atomic operations configure test explicitly disabled when
compiling with Sun C.

Previously, the source IP address of a response UDP datagram could differ from
the original datagram destination address.  This could happen if the server UDP
socket is bound to a wildcard address and the network interface chosen to output
the response packet has a different default address than the destination address
of the original packet.  For example, if two addresses from the same network are
configured on an interface.

Now source address is set explicitly if a response is sent for a server UDP
socket bound to a wildcard address.

This change allows setting the destination IPv6 address of a UDP datagram
received on a wildcard socket.