Move WEBRTC_BUILD_LIBEVENT definition to base/BUILD.gn and base/base.gyp. This use all_dependent_co…

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index e7ed156ade9d9cbf12591db69791af8137d6d280..bcc6ad901e319a4245003df45d6be908cd9e3d21 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -187,6 +187,10 @@ static_library("rtc_base_approved") {
   }
 }
 
+config("enable_libevent_config") {
+  defines = [ "WEBRTC_BUILD_LIBEVENT" ]
+}
+
 static_library("rtc_task_queue") {
   public_deps = [
     ":rtc_base_approved",
@@ -212,6 +216,7 @@ static_library("rtc_task_queue") {
       "task_queue_libevent.cc",
       "task_queue_posix.cc",
     ]
+    all_dependent_configs = [ ":enable_libevent_config" ]
   } else {
     if (is_mac || is_ios) {
       sources += [