paths.h: add _BSD_SOURCE guard back, also check for _GNU_SOURCE

So things (like m4) expecting paths.h as part of glibc will get
what they want with _GNU_SOURCE.

headers/compatibility/bsd/paths.h

@@ -38,6 +38,9 @@
 #define	_PATHS_H_
 
 
+#if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
+
+
 #include <sys/cdefs.h>
 
 /* Default search path. */
@@ -138,4 +141,7 @@ __END_DECLS
 #endif /* RESCUE */
 
 
+#endif /* _BSD_SOURCE || _GNU_SOURCE */
+
+
 #endif /* !_PATHS_H_ */