diff --git a/glib/gmain.c b/glib/gmain.c
index 933c04f26..78de717bf 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -31,6 +31,11 @@
  * MT safe
  */
 
+/* for pipe2; need to define it first to avoid
+ * other headers pulling in unistd.h
+ */
+#define _GNU_SOURCE
+
 #include "config.h"
 #include "glibconfig.h"
 
@@ -47,8 +52,6 @@
 #define G_MAIN_POLL_DEBUG
 #endif
 
-#define _GNU_SOURCE  /* for pipe2 */
-
 #include <signal.h>
 #include <sys/types.h>
 #include <time.h>