Check __GLIBC_PREREQ availability before use.

BUG=webrtc:7287

Review-Url: https://codereview.webrtc.org/2727783004
Cr-Commit-Position: refs/heads/master@{#17070}
diff --git a/webrtc/system_wrappers/source/cpu_features_linux.c b/webrtc/system_wrappers/source/cpu_features_linux.c
index f23d827..112ad03 100644
--- a/webrtc/system_wrappers/source/cpu_features_linux.c
+++ b/webrtc/system_wrappers/source/cpu_features_linux.c
@@ -11,6 +11,9 @@
 #include <stdlib.h>
 #include <string.h>
 #include <features.h>
+#ifndef __GLIBC_PREREQ
+#define __GLIBC_PREREQ(a, b) 0
+#endif
 #if __GLIBC_PREREQ(2, 16)
 #include <sys/auxv.h>
 #else