diff --git a/evdns.c b/evdns.c
index 11e25254..ffb10041 100644
--- a/evdns.c
+++ b/evdns.c
@@ -76,7 +76,9 @@
 #include <openssl/rand.h>
 #endif
 
+#ifndef _FORTIFY_SOURCE
 #define _FORTIFY_SOURCE 3
+#endif
 
 #include <string.h>
 #include <fcntl.h>