Subject: [PATCH] libwebsockets.h: use _GNU_SOURCE instead of __USE_GNU

this collides with use of features.h which says '#define __USE_GNU 1'

see also second answer:
http://stackoverflow.com/questions/7296963/gnu-source-and-use-gnu?answertab=active#tab-top
diff --git a/lib/libwebsockets.h b/lib/libwebsockets.h
index 16f294f..2fd78b1 100644
--- a/lib/libwebsockets.h
+++ b/lib/libwebsockets.h
@@ -65,7 +65,9 @@
 #else // NOT WIN32
 
 /* to get ppoll() */
-#define __USE_GNU
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <poll.h>
 #include <unistd.h>