fix _ALL_SOURCE logic to avoid possible redefinition of _GNU_SOURCE

this could be an error if _GNU_SOURCE was already defined differently
by the application.

include/features.h

@@ -1,7 +1,7 @@
 #ifndef _FEATURES_H
 #define _FEATURES_H
 
-#ifdef _ALL_SOURCE
+#if defined(_ALL_SOURCE) && !defined(_GNU_SOURCE)
 #define _GNU_SOURCE 1
 #endif