Enabled IPV6_RECVPKTINFO / IPV6_PKTINFO on macOS.

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

auto/os/darwin

diff --git a/auto/os/darwin b/auto/os/darwin
index b4b3ad3..429468f 100644
--- a/auto/os/darwin
+++ b/auto/os/darwin
@@ -17,6 +17,9 @@
 MAIN_LINK=
 MODULE_LINK="-shared -Wl,-undefined,dynamic_lookup"
 
+CC_AUX_FLAGS="$CC_AUX_FLAGS -D__APPLE_USE_RFC_3542"
+
+
 # kqueue
 
 echo " + kqueue found"
@@ -86,7 +89,6 @@
 
 # sendfile()
 
-CC_AUX_FLAGS="$CC_AUX_FLAGS"
 ngx_feature="sendfile()"
 ngx_feature_name="NGX_HAVE_SENDFILE"
 ngx_feature_run=yes

src/os/unix/ngx_darwin_config.h

diff --git a/src/os/unix/ngx_darwin_config.h b/src/os/unix/ngx_darwin_config.h
index cfe3ce2..0dfe633 100644
--- a/src/os/unix/ngx_darwin_config.h
+++ b/src/os/unix/ngx_darwin_config.h
@@ -9,6 +9,9 @@
 #define _NGX_DARWIN_CONFIG_H_INCLUDED_
 
 
+#define __APPLE_USE_RFC_3542    /* IPV6_PKTINFO */
+
+
 #include <sys/types.h>
 #include <sys/time.h>
 #include <unistd.h>