diff --git a/include/afl-fuzz.h b/include/afl-fuzz.h
index 9da5cc03..27668da0 100644
--- a/include/afl-fuzz.h
+++ b/include/afl-fuzz.h
@@ -31,7 +31,7 @@
 #define MESSAGES_TO_STDOUT
 
 #ifndef _GNU_SOURCE
-  #define _GNU_SOURCE 1
+  #define _GNU_SOURCE
 #endif
 #ifndef _FILE_OFFSET_BITS
   #define _FILE_OFFSET_BITS 64
diff --git a/include/android-ashmem.h b/include/android-ashmem.h
index 1bfd3220..065c213b 100644
--- a/include/android-ashmem.h
+++ b/include/android-ashmem.h
@@ -2,7 +2,9 @@
   #ifndef _ANDROID_ASHMEM_H
     #define _ANDROID_ASHMEM_H
 
-    #define _GNU_SOURCE
+    #ifndef _GNU_SOURCE
+      #define _GNU_SOURCE
+    #endif
     #include <sys/syscall.h>
     #include <unistd.h>
     #include <fcntl.h>
diff --git a/src/afl-ld-lto.c b/src/afl-ld-lto.c
index b306c8d5..b1e6c848 100644
--- a/src/afl-ld-lto.c
+++ b/src/afl-ld-lto.c
@@ -23,7 +23,9 @@
 */
 
 #define AFL_MAIN
-#define _GNU_SOURCE
+#ifndef _GNU_SOURCE
+  #define _GNU_SOURCE
+#endif
 
 #include "config.h"
 #include "types.h"