Revert "Revert "Upgrade to 2.1.8-stable (2017-01-22)." and "Probably Mac build fix?""

This reverts commit 83a0c9c65a60a92d3ea5542596b3ba56db492c37.

Bug: 64543673
Test: make checkbuild
Test: Manual tombstoned test
Change-Id: I84bb128d1dec433195f2cbdbf70236ba17fa9955
diff --git a/ipv6-internal.h b/ipv6-internal.h
index e373711..0c20737 100644
--- a/ipv6-internal.h
+++ b/ipv6-internal.h
@@ -27,14 +27,16 @@
 /* Internal use only: Fake IPv6 structures and values on platforms that
  * do not have them */
 
-#ifndef _EVENT_IPV6_INTERNAL_H
-#define _EVENT_IPV6_INTERNAL_H
+#ifndef IPV6_INTERNAL_H_INCLUDED_
+#define IPV6_INTERNAL_H_INCLUDED_
+
+#include "event2/event-config.h"
+#include "evconfig-private.h"
 
 #include <sys/types.h>
-#ifdef _EVENT_HAVE_SYS_SOCKET_H
+#ifdef EVENT__HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 #endif
-#include "event2/event-config.h"
 #include "event2/util.h"
 
 #ifdef __cplusplus
@@ -47,17 +49,17 @@
  * properly.
  */
 
-#ifndef _EVENT_HAVE_STRUCT_IN6_ADDR
+#ifndef EVENT__HAVE_STRUCT_IN6_ADDR
 struct in6_addr {
 	ev_uint8_t s6_addr[16];
 };
 #endif
 
-#ifndef _EVENT_HAVE_SA_FAMILY_T
+#ifndef EVENT__HAVE_SA_FAMILY_T
 typedef int sa_family_t;
 #endif
 
-#ifndef _EVENT_HAVE_STRUCT_SOCKADDR_IN6
+#ifndef EVENT__HAVE_STRUCT_SOCKADDR_IN6
 struct sockaddr_in6 {
 	/* This will fail if we find a struct sockaddr that doesn't have
 	 * sa_family as the first element. */