diff --git a/src/dump.c b/src/dump.c
index d87d5c7..eb596aa 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 d60a93e..8ec57ba 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 b9190c2..d429c91 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 d0517d5..74e8064 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>