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/bison.m4

@@ -22,12 +22,17 @@
 ## Identification.  ##
 ## ---------------- ##
 
+# b4_generated_by
+# ---------------
+m4_define([b4_generated_by],
+[b4_comment([A Bison parser, made by GNU Bison b4_version.])
+])
+
 # b4_copyright(TITLE, [YEARS])
 # ----------------------------
 # If YEARS are not defined, use b4_copyright_years.
 m4_define([b4_copyright],
-[b4_comment([A Bison parser, made by GNU Bison b4_version.])
-
+[b4_generated_by
 b4_comment([$1
 
 ]m4_dquote(m4_text_wrap([Copyright (C)

data/location.cc

@@ -322,6 +322,7 @@ m4_define([b4_location_define],
 
 m4_ifdef([b4_position_file], [[
 ]b4_output_begin([b4_dir_prefix], [b4_position_file])[
+]b4_generated_by[
 // Starting with Bison 3.2, this file is useless: the structure it
 // used to define is now defined in "]b4_location_file[".
 //

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.
 //