c++: provide control over the stack.hh file name

It was not a good idea to generate the file stack.hh.  It never was.
But now we have to deal with backward compatibility: if we stop
generating it, the build system of some build system will probably
break.

So offer the user a means to (i) decide what the name of the output
file should be, and (ii) not generate this file at all (its content
will be inline where the parser is defined).

* data/lalr1.cc (b4_percent_define_check_file_complain)
(b4_percent_define_check_file): New.
* data/stack.hh: Generate the file only if api.stack.file is not
empty.
In that case, use it as file name.
* data/lalr1.cc: Adjust to include the right file, or to include
the definition of stack.
* tests/calc.at, tests/output.at: Exercise api.stack.file.

data/lalr1.cc

@@ -20,6 +20,7 @@ m4_include(b4_pkgdatadir/[c++.m4])
 # api.value.type=variant is valid.
 m4_define([b4_value_type_setup_variant])
 
+
 # b4_integral_parser_table_declare(TABLE-NAME, CONTENT, COMMENT)
 # --------------------------------------------------------------
 # Declare "parser::yy<TABLE-NAME>_" whose contents is CONTENT.
@@ -162,8 +163,9 @@ m4_define([b4_shared_declarations],
 # include <vector>
 
 ]b4_cxx_portability[
+]m4_ifdef([b4_stack_file],
+          [[# include "]b4_stack_file["]])[
 ]b4_defines_if([[
-# include "stack.hh"
 ]b4_bison_locations_if([[# include "location.hh"]])])[
 ]b4_variant_if([b4_variant_includes])[
 
@@ -174,9 +176,9 @@ m4_define([b4_shared_declarations],
 
 ]b4_namespace_open[
 
+]m4_ifdef([b4_stack_file], [], [b4_stack_define])[
 ]b4_defines_if([],
-[b4_stack_define
-b4_bison_locations_if([b4_position_define
+[b4_bison_locations_if([b4_position_define
 b4_location_define])])[
 
 ]b4_variant_if([b4_variant_define])[