diff --git a/src/dump.c b/src/dump.c
index bbf87d0..6a7523f 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -5,7 +5,10 @@
  * it under the terms of the MIT license. See LICENSE for details.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/src/load.c b/src/load.c
index 2cc7a5f..1eb3be4 100644
--- a/src/load.c
+++ b/src/load.c
@@ -5,7 +5,10 @@
  * it under the terms of the MIT license. See LICENSE for details.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include <errno.h>
 #include <limits.h>
 #include <stdio.h>
diff --git a/src/strbuffer.c b/src/strbuffer.c
index 6d4edd6..b806615 100644
--- a/src/strbuffer.c
+++ b/src/strbuffer.c
@@ -5,7 +5,10 @@
  * it under the terms of the MIT license. See LICENSE for details.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
+
 #include <stdlib.h>
 #include <string.h>
 #include "jansson_private.h"
diff --git a/src/value.c b/src/value.c
index 003b5f3..261c0f1 100644
--- a/src/value.c
+++ b/src/value.c
@@ -5,7 +5,9 @@
  * it under the terms of the MIT license. See LICENSE for details.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include <stddef.h>
 #include <stdlib.h>