c++: always issue the "generated by" message

Some users rely on this sentence to know that the file can be ignored.
Reported by Alexandre Duret-Lutz.

* data/bison.m4 (b4_generated_by): New.
(b4_copyright): Use it.
* data/location.cc, data/stack.hh: Use it too, for the stub files
(position.hh and stack.hh).

data/stack.hh

@@ -148,6 +148,7 @@ m4_define([b4_stack_define],
 
 m4_ifdef([b4_stack_file],
 [b4_output_begin([b4_dir_prefix], [b4_stack_file])[
+]b4_generated_by[
 // Starting with Bison 3.2, this file is useless: the structure it
 // used to define is now defined with the parser itself.
 //