Separate building and enabling libevent.

webrtc/base/base.gyp

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 {
   'includes': [ '../build/common.gypi', ],
@@ skipping 103 matching lines @@
           'sources': [
             'logging.cc',
             'logging.h',
             'logging_mac.mm',
           ],
         }],
         ['build_libevent==1', {
           'dependencies': [
             '<(DEPTH)/base/third_party/libevent/libevent.gyp:libevent',
           ],
-        }, {
+        }],
+        ['enable_libevent!=1', {

[kjellander_webrtc, 2016/05/16 12:21:13]

I suggest starting with 
enable_libevent==1

and then put this in the else-clause, i.e. after
}, {

Also please rewrite the source listings without using sources! exclusions - it
makes GYP really hard to read and maintain (or ask tommi to do so in a separate
CL; IMO it should never have been allowed to land like that in
https://codereview.webrtc.org/1984503002).

[phoglund, 2016/05/16 13:18:01]

Yeah, I might as well do it here so I'll do it.

           'sources!': [ 'task_queue_libevent.cc' ],
           'conditions': [
             ['OS=="linux" or OS=="android"', {
               'sources!': [ 'task_queue_posix.cc' ],
             }],
           ],
         }],
-        ['build_libevent==1 or OS=="linux" or OS=="android" or OS=="win"', {
+        ['enable_libevent==1 or OS=="linux" or OS=="android" or OS=="win"', {

[kjellander_webrtc, 2016/05/16 12:21:13]

See previous comment. You can put this into a sub-condition of the suggested
if-else construction above (for readability).

[phoglund, 2016/05/16 13:18:01]

Done.

           'sources!': [ 'task_queue_gcd.cc' ],
         }],
         ['OS=="mac" and build_with_chromium==0', {
           'all_dependent_settings': {
             'xcode_settings': {
               'OTHER_LDFLAGS': [
                 # needed for logging_mac.mm
                 '-framework Foundation',
               ],
             },
@@ skipping 478 matching lines @@
     },
     {
      'target_name': 'gtest_prod',
       'type': 'static_library',
       'sources': [
         'gtest_prod_util.h',
       ],
     },
   ],
 }