The support first appeared in OS X Mavericks 10.9 and documented since
OS X Yosemite 10.10.

It has a subtle implementation difference from other operating systems
in that the TCP_KEEPALIVE socket option (used in place of TCP_KEEPIDLE)
isn't inherited from a listening socket to an accepted socket.

An apparent reason for this behaviour is that it might be preserved for
the sake of backward compatibility.  The TCP_KEEPALIVE socket option is
not inherited since appearance in OS X Panther 10.3, which long predates
two other TCP_KEEPINTVL and TCP_KEEPCNT socket options.

Thanks to Andy Pan for initial work.

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

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

The auto/module script is extended to understand ngx_module_link=DYNAMIC.
When set, it links the module as a shared object rather than statically
into nginx binary.  The module can later be loaded using the "load_module"
directive.

New auto/module parameter ngx_module_order allows to define module loading
order in complex cases.  By default the order is set based on ngx_module_type.

3rd party modules can be compiled dynamically using the --add-dynamic-module
configure option, which will preset ngx_module_link to "DYNAMIC" before
calling the module config script.

Win32 support is rudimentary, and only works when using MinGW gcc (which
is able to handle exports/imports automatically).

In collaboration with Ruslan Ermilov.

The auto/feature call above is enough to set NGX_HAVE_SENDFILE.

*) Darwin sendfile() support