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.

Signed-off-by: Piotr Sikora <piotr@aviatrix.com>

Based on a patch by Piotr Sikora.

Previously, the resulting binary was successfully linked
because libcrypt was added in a separate test for crypt().

Patch by Piotr Sikora.

In nginx source code the inttypes.h include, if available, is used to define
standard integer types.  Changed the SO_COOKIE configure test to follow this.

In particular, this sets a reasonable cacheline size on FreeBSD and macOS,
which prefer to use this name and both lack _SC_LEVEL1_DCACHE_LINESIZE.