Fix macro clash with _USE_MATH_DEFINES.

Bug: chromium:788675
Change-Id: I4840fd013a81ffe157323b0bb876d64fd60d8a19
Reviewed-on: https://webrtc-review.googlesource.com/32304
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21235}
diff --git a/modules/audio_processing/include/audio_processing.h b/modules/audio_processing/include/audio_processing.h
index b750c69..ab9d2cd 100644
--- a/modules/audio_processing/include/audio_processing.h
+++ b/modules/audio_processing/include/audio_processing.h
@@ -12,7 +12,9 @@
 #define MODULES_AUDIO_PROCESSING_INCLUDE_AUDIO_PROCESSING_H_
 
 // MSVC++ requires this to be set before any other includes to get M_PI.
+#ifndef _USE_MATH_DEFINES
 #define _USE_MATH_DEFINES
+#endif
 
 #include <math.h>
 #include <stddef.h>  // size_t