diff --git a/sysdep/linux/syspriv.h b/sysdep/linux/syspriv.h
index 51bef731..8b210f06 100644
--- a/sysdep/linux/syspriv.h
+++ b/sysdep/linux/syspriv.h
@@ -1,6 +1,10 @@
 #ifndef _BIRD_SYSPRIV_H_
 #define _BIRD_SYSPRIV_H_
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <unistd.h>
 #include <sys/prctl.h>
 #include <linux/capability.h>
diff --git a/sysdep/unix/io.c b/sysdep/unix/io.c
index 78faa186..20a40547 100644
--- a/sysdep/unix/io.c
+++ b/sysdep/unix/io.c
@@ -9,7 +9,9 @@
 
 /* Unfortunately, some glibc versions hide parts of RFC 3542 API
    if _GNU_SOURCE is not defined. */
-#define _GNU_SOURCE 1
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/sysdep/unix/main.c b/sysdep/unix/main.c
index 938190ef..eb33a566 100644
--- a/sysdep/unix/main.c
+++ b/sysdep/unix/main.c
@@ -7,7 +7,7 @@
  */
 
 #ifndef _GNU_SOURCE
-#define _GNU_SOURCE 1
+#define _GNU_SOURCE
 #endif
 
 #include <stdio.h>
diff --git a/sysdep/unix/main_helper.c b/sysdep/unix/main_helper.c
index 0af7b80a..7b71bed1 100644
--- a/sysdep/unix/main_helper.c
+++ b/sysdep/unix/main_helper.c
@@ -9,7 +9,7 @@
 #undef LOCAL_DEBUG
 
 #ifndef _GNU_SOURCE
-#define _GNU_SOURCE 1
+#define _GNU_SOURCE
 #endif
 
 #include <stdio.h>
diff --git a/sysdep/unix/main_helper.h b/sysdep/unix/main_helper.h
index 3beea1dc..1e991a22 100644
--- a/sysdep/unix/main_helper.h
+++ b/sysdep/unix/main_helper.h
@@ -9,6 +9,10 @@
 #ifndef _BIRD_MAIN_HELPER_H_
 #define _BIRD_MAIN_HELPER_H_
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include "lib/birdlib.h"
 #include "lib/socket.h"
 #include "sysdep/config.h"