Separate building and enabling libevent.

webrtc/base/base.gyp

Index: webrtc/base/base.gyp
diff --git a/webrtc/base/base.gyp b/webrtc/base/base.gyp
index 4564504aabf01390c1a12829b73a7145b53f0744..c0e92d94685b70ccdeba41ae006827d9f23f6b3a 100644
--- a/webrtc/base/base.gyp
+++ b/webrtc/base/base.gyp
@@ -121,7 +121,8 @@
           '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"', {
@@ -129,7 +130,7 @@
             }],
           ],
         }],
-        ['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', {