Separate building and enabling libevent.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index aefcfc5a4990ee3e4a7eb7b7d5a6b4862390b34b..41b52d5f1702f74c98defbf387ea715589da62f0 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -148,11 +148,7 @@ static_library("rtc_base_approved") {
     "systeminfo.cc",
     "systeminfo.h",
     "task_queue.h",
-    "task_queue_gcd.cc",
-    "task_queue_libevent.cc",
-    "task_queue_posix.cc",
     "task_queue_posix.h",
-    "task_queue_win.cc",
     "template_util.h",
     "thread_annotations.h",
     "thread_checker.h",
@@ -182,18 +178,21 @@ static_library("rtc_base_approved") {
   if (!is_win && !is_mac && !is_ios && !is_nacl) {

[kjellander_webrtc, 2016/05/16 15:09:12]

Can you please introduce and use rtc_build_libevent and rtc_enable_libevent
variables in webrtc/build/webrtc.gni?

[phoglund, 2016/05/16 15:32:29]

Done.

     deps += [ "//base/third_party/libevent" ]
     defines += [ "WEBRTC_BUILD_LIBEVENT" ]

[kjellander_webrtc, 2016/05/16 15:09:12]

The define should got into
webrtc/BUILD.gn's common_config target?
reflecting the change in
https://codereview.webrtc.org/1984503002/diff/280001/webrtc/build/common.gypi

[phoglund, 2016/05/16 15:32:29]

I think that feels icky. The defines there are very general, and I think it's
wise to keep such a specific define close to where it's being used.

[kjellander_webrtc, 2016/05/16 15:34:28]

I agree. Can you move the other one into base.gyp instead then? The important
thing is that GN and GYP are in sync per target (or we'll have a hard time when
we're going to switch).

[phoglund, 2016/05/16 15:44:44]

Certainly, that's a good idea.

-  }
-
-  if (is_mac || is_ios || is_win || is_nacl) {
-    sources -= [ "task_queue_libevent.cc" ]
-  }
-
-  if (is_linux || is_android || is_win || is_nacl) {
-    sources -= [ "task_queue_gcd.cc" ]
-  }
-
-  if (is_nacl) {
-    sources -= [ "task_queue_posix.cc" ]
+    sources += [
+      "task_queue_libevent.cc",
+      "task_queue_posix.cc",
+    ]
+  } else {
+    # If not libevent, fall back to the other task queues.
+    if (is_mac || is_ios) {
+      sources += [
+        "task_queue_gcd.cc",
+        "task_queue_posix.cc",
+      ]
+    }
+    if (is_win) {
+      sources += [ "task_queue_win.cc" ]
+    }
   }
 }