[libgsystem] Only #define _GNU_SOURCE if it hasn't already been

[Colin Walters <walters src gnome org>]

commit 173325b97e1e77082b147f1330f758e5a82ae3a9
Author: Colin Walters <walters verbum org>
Date:   Wed Oct 3 13:14:38 2012 -0400

    Only #define _GNU_SOURCE if it hasn't already been
    
    NetworkManager already defines it, so let's not error out.

 gsystem-file-utils.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gsystem-file-utils.c b/gsystem-file-utils.c
index 3064616..92ddf49 100644
--- a/gsystem-file-utils.c
+++ b/gsystem-file-utils.c
@@ -21,7 +21,9 @@
 
 #include "config.h"
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 
 #include "libgsystem.h"
 #include <glib/gstdio.h>