* include/features.h: Add comment to explain what's going to happen

here at one point.
	* include/sys/stdio.h: Guard getline and getdelim prototypes with
	_GNU_SOURCE to avoid collision with old-style declarations.

winsup/cygwin/include/sys/stdio.h

@@ -28,8 +28,10 @@ details. */
 
 __BEGIN_DECLS
 
+#ifdef _GNU_SOURCE
 ssize_t	_EXFUN(getline, (char **, size_t *, FILE *));
 ssize_t	_EXFUN(getdelim, (char **, size_t *, int, FILE *));
+#endif /* _GNU_SOURCE */
 
 __END_DECLS