Enable the -Wundef warning for clang

webrtc/base/event.cc

Index: webrtc/base/event.cc
diff --git a/webrtc/base/event.cc b/webrtc/base/event.cc
index 0e267dc1ac31a0743613650233229a0e6a12e6d0..4b348f63a1682585e419bb9518dc3aa8d4f99031 100644
--- a/webrtc/base/event.cc
+++ b/webrtc/base/event.cc
@@ -86,7 +86,7 @@ bool Event::Wait(int milliseconds) {
     // Converting from seconds and microseconds (1e-6) plus
     // milliseconds (1e-3) to seconds and nanoseconds (1e-9).
 
-#if HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
+#ifdef HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
     // Use relative time version, which tends to be more efficient for
     // pthread implementations where provided (like on Android).
     ts.tv_sec = milliseconds / 1000;
@@ -109,7 +109,7 @@ bool Event::Wait(int milliseconds) {
   pthread_mutex_lock(&event_mutex_);
   if (milliseconds != kForever) {
     while (!event_status_ && error == 0) {
-#if HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
+#ifdef HAVE_PTHREAD_COND_TIMEDWAIT_RELATIVE
       error = pthread_cond_timedwait_relative_np(
           &event_cond_, &event_mutex_, &ts);
 #else