OpenSSL now uses pthread_atfork(), and this requires -lpthread on Linux
to compile.  Introduced NGX_LIBPTHREAD to add it as appropriate, similar
to existing NGX_LIBDL.

When clock_gettime(CLOCK_MONOTONIC) (or faster variants, _FAST on FreeBSD,
and _COARSE on Linux) is available, we now use it for ngx_current_msec.
This should improve handling of timers if system time changes (ticket #189).

Determine cacheline size at runtime if supported
using sysconf(_SC_LEVEL1_DCACHE_LINESIZE). In case not supported,
fallback to compile time defaults.

In 2c7b488a61fb, IP_BIND_ADDRESS_NO_PORT test was accidentally placed
between SO_BINDANY, IP_TRANSPARENT, and IP_BINDANY tests.  Moved it after
these tests.

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.

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.

IPv6 now compiled-in automatically if support is found.  If there is a need
to disable it for some reason, --with-cc-opt="-DNGX_HAVE_INET6=0" can be used
for this.