Fix build with CFLAGS="-std=c11 -D_GNU_SOURCE"

BUG=chromium:427584

Change-Id: If134b2882c6b0cb97a2ee844748213524f778101

aom/src/aom_encoder.c

diff --git a/aom/src/aom_encoder.c b/aom/src/aom_encoder.c
index 22765d6..37b9495 100644
--- a/aom/src/aom_encoder.c
+++ b/aom/src/aom_encoder.c
@@ -16,7 +16,9 @@
 #include "config/aom_config.h"
 
 #if HAVE_FEXCEPT
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <fenv.h>
 #endif