Allow using system libevent instead of the bundled one.

third_party/libevent/libevent.gyp

 # Copyright (c) 2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
-  'targets': [
-    {
-      'target_name': 'libevent',
-      'product_name': 'event',
-      'type': '<(library)',
-      'sources': [
-        'buffer.c',
-        'evbuffer.c',
-        'evdns.c',
-        'event.c',
-        'event_tagging.c',
-        'evrpc.c',
-        'evutil.c',
-        'http.c',
-        'log.c',
-        'poll.c',
-        'select.c',
-        'signal.c',
-        'strlcpy.c',
+  'variables': {
+    'use_system_libevent%': 0,
+  },
+  'conditions': [
+    ['use_system_libevent==0', {
+      'targets': [
+        {
+          'target_name': 'libevent',
+          'product_name': 'event',
+          'type': '<(library)',
+          'sources': [
+            'buffer.c',
+            'evbuffer.c',
+            'evdns.c',
+            'event.c',
+            'event_tagging.c',
+            'evrpc.c',
+            'evutil.c',
+            'http.c',
+            'log.c',
+            'poll.c',
+            'select.c',
+            'signal.c',
+            'strlcpy.c',
+          ],
+          'defines': [
+            'HAVE_CONFIG_H',
+          ],
+          'include_dirs': [
+            '.',   # libevent includes some of its own headers with
+                   # #include <...> instead of #include "..."
+          ],
+          'conditions': [
+            # libevent has platform-specific implementation files.  Since its
+            # native build uses autoconf, platform-specific config.h files are
+            # provided and live in platform-specific directories.
+            [ 'OS == "linux"', {
+              'sources': [ 'epoll.c', 'epoll_sub.c' ],
+              'include_dirs': [ 'linux' ],
+              'link_settings': {
+                'libraries': [
+                  # We need rt for clock_gettime().
+                  # TODO(port) Maybe on FreeBSD as well?
+                  '-lrt',
+                ],
+              },
+            }],
+            [ 'OS == "mac" or OS == "freebsd"', {
+              'sources': [ 'kqueue.c' ],
+              'include_dirs': [ 'mac' ]
+            }],
+          ],
+        },
       ],
-      'defines': [
-        'HAVE_CONFIG_H',
-      ],
-      'include_dirs': [
-        '.',   # libevent includes some of its own headers with #include <...>
-               # instead of #include "..."
-      ],
-      'conditions': [
-        # libevent has platform-specific implementation files.  Since its
-        # native build uses autoconf, platform-specific config.h files are
-        # provided and live in platform-specific directories.
-        [ 'OS == "linux"', {
-          'sources': [ 'epoll.c', 'epoll_sub.c' ],
-          'include_dirs': [ 'linux' ],
+    }, {  # use_system_libevent != 0
+      'targets': [
+        {
+          'target_name': 'libevent',
+          'type': 'settings',
+          'direct_dependent_settings': {
+            'defines': [
+              'USE_SYSTEM_LIBEVENT',
+            ],
+          },
           'link_settings': {
             'libraries': [
-              # We need rt for clock_gettime().
-              # TODO(port) Maybe on FreeBSD as well?
-              '-lrt',
+              '-levent',
             ],
           },
-        }],
-        [ 'OS == "mac" or OS == "freebsd"', {
-          'sources': [ 'kqueue.c' ],
-          'include_dirs': [ 'mac' ]
-        }],
+        }
       ],
-    },
+    }],
   ],
 }
 
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