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..1e868b504e528e911118efd52f261bf14129fff9 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",
@@ -179,21 +175,25 @@ static_library("rtc_base_approved") {
     ]
   }
 
-  if (!is_win && !is_mac && !is_ios && !is_nacl) {
+  if (rtc_build_libevent) {
     deps += [ "//base/third_party/libevent" ]
-    defines += [ "WEBRTC_BUILD_LIBEVENT" ]
   }
-
-  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" ]
+  if (rtc_enable_libevent) {
+    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" ]
+    }
   }
 }