[babl] babl-mutex: Define _GNU_SOURCE instead of __USE_GNU

[Jon Nordby <jonnor src gnome org>]

commit 70244fcf5385a657790099b162f97539eb9b14c6
Author: Jon Nordby <jononor gmail com>
Date:   Wed Mar 18 12:07:01 2015 +0100

    babl-mutex: Define _GNU_SOURCE instead of __USE_GNU
    
    __USE_GNU should only be set internally by the libc feature detect macros.
    On older Debian-based systems, as used by Heroku cedar-10 stack, this failed
    to provide the definition of PTHREAD_MUTEX_RECURSIVE

 babl/babl-mutex.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/babl/babl-mutex.h b/babl/babl-mutex.h
index 99fdbaa..57154b3 100644
--- a/babl/babl-mutex.h
+++ b/babl/babl-mutex.h
@@ -20,7 +20,7 @@
 #define _BABL_MUTEX_H
 
 #ifndef _WIN32
-#define __USE_GNU 1
+#define __GNU_SOURCE 1
 #include <pthread.h>
 #else
 #include <windows.h>