all: avoid useless comments and #lines

Currently we emit useless code for places where we might issue user
content, but there is none.  This commit avoids this.  Besides, some
of the comments looked like implementation details ("Copy the first
part of user declarations"), rather than made for the reader of the
result ("First part of user prologue").

On Bison's parse-gram.c we get:

    @@ -76,10 +76,6 @@
     #define yynerrs         gram_nerrs

    -/* Copy the first part of user declarations.  */
    -
    -#line 82 "src/parse-gram.c" /* yacc.c:339  */
    -

* data/bison.m4 (b4_define_user_code): Accept a comment to document
the section.
Do not emit any code if the content is empty.
Adjust callers to not emit the comment.
Do not
* data/glr.c, data/glr.cc, data/lalr1.cc, data/lalr1.java, data/yacc.c:
Adjust.

data/bison.m4

@@ -613,13 +613,15 @@ m4_define([b4_user_code],
 b4_syncline([@oline@], [@ofile@])])
 
 
-# b4_define_user_code(MACRO)
-# --------------------------
-# From b4_MACRO, build b4_user_MACRO that includes the synclines.
+# b4_define_user_code(MACRO, COMMENT)
+# -----------------------------------
+# From b4_MACRO, if defined, build b4_user_MACRO that includes the synclines.
 m4_define([b4_define_user_code],
 [m4_define([b4_user_$1],
-[b4_user_code([b4_$1])])])
-
+           [m4_ifdef([b4_$1],
+                     [m4_ifval([$2],
+                               [b4_comment([$2])
+])b4_user_code([b4_$1])])])])
 
 # b4_user_actions
 # b4_user_initial_action
@@ -629,9 +631,9 @@ m4_define([b4_define_user_code],
 # ----------------------
 # Macros that issue user code, ending with synclines.
 b4_define_user_code([actions])
-b4_define_user_code([initial_action])
-b4_define_user_code([post_prologue])
-b4_define_user_code([pre_prologue])
+b4_define_user_code([initial_action], [User initialization code.])
+b4_define_user_code([post_prologue], [Second part of user prologue.])
+b4_define_user_code([pre_prologue], [First part of user prologue.])
 b4_define_user_code([union_members])
 
 
@@ -1056,8 +1058,6 @@ b4_check_user_names_wrap([[code]], [[qualifier]])
 ## ---------------- ##
 
 # m4_define_default([b4_lex_param], [])   dnl breaks other skeletons
-m4_define_default([b4_pre_prologue], [])
-m4_define_default([b4_post_prologue], [])
 m4_define_default([b4_epilogue], [])
 m4_define_default([b4_parse_param], [])
 

data/glr.c

@@ -236,7 +236,6 @@ b4_copyright([Skeleton implementation for Bison GLR parsers in C],
 #define yynerrs ]b4_prefix[nerrs]b4_locations_if([[
 #define yylloc  ]b4_prefix[lloc]])]))[
 
-/* First part of user declarations.  */
 ]b4_user_pre_prologue[
 
 ]b4_null_define[
@@ -259,9 +258,8 @@ b4_copyright([Skeleton implementation for Bison GLR parsers in C],
 static YYSTYPE yyval_default;]b4_locations_if([[
 static YYLTYPE yyloc_default][]b4_yyloc_default;])[
 
-/* Copy the second part of user declarations.  */
-]b4_user_post_prologue
-b4_percent_code_get[]dnl
+]b4_user_post_prologue[
+]b4_percent_code_get[]dnl
 
 [#include <stdio.h>
 #include <stdlib.h>
@@ -820,7 +818,7 @@ yyuserAction (yyRuleNum yyn, size_t yyrhslen, yyGLRStackItem* yyvsp,
 ]])[
   switch (yyn)
     {
-      ]b4_user_actions[
+]b4_user_actions[
       default: break;
     }
 
@@ -2266,7 +2264,6 @@ yyrecoverSyntaxError (yyGLRStack* yystackp]b4_user_formals[)
   yylloc = yyloc_default;])[
 ]m4_ifdef([b4_initial_action], [
 b4_dollar_pushdef([yylval], [], [yylloc])dnl
-  /* User initialization code.  */
   b4_user_initial_action
 b4_dollar_popdef])[]dnl
 [

data/glr.cc

@@ -44,7 +44,7 @@
 #   filename member).
 
 # We require a pure interface.
-m4_define([b4_pure_flag],      [1])
+m4_define([b4_pure_flag], [1])
 
 m4_include(b4_pkgdatadir/[c++.m4])
 b4_bison_locations_if([m4_include(b4_pkgdatadir/[location.cc])])

data/lalr1.cc

@@ -418,13 +418,11 @@ m4_if(b4_prefix, [yy], [],
 // Take the name prefix into account.
 [#]define yylex   b4_prefix[]lex])[
 
-// First part of user declarations.
 ]b4_user_pre_prologue[
 
 ]b4_defines_if([[#include "@basename(]b4_spec_defines_file[@)"]],
                [b4_shared_declarations([cc])])[
 
-// User implementation prologue.
 ]b4_user_post_prologue[
 ]b4_percent_code_get[
 
@@ -768,7 +766,6 @@ m4_if(b4_prefix, [yy], [],
 
 ]m4_ifdef([b4_initial_action], [
 b4_dollar_pushdef([yyla.value], [], [yyla.location])dnl
-    // User initialization code.
     b4_user_initial_action
 b4_dollar_popdef])[]dnl
 

data/lalr1.java

@@ -87,10 +87,10 @@ b4_copyright([Skeleton implementation for Bison LALR(1) parsers in Java],
              [2007-2015, 2018])
 
 b4_percent_define_ifdef([package], [package b4_percent_define_get([package]);
-])[/* First part of user declarations.  */
-]b4_user_pre_prologue
-b4_user_post_prologue
-b4_percent_code_get([[imports]])
+])[
+]b4_user_pre_prologue[
+]b4_user_post_prologue[
+]b4_percent_code_get([[imports]])
 [/**
  * A Bison parser, automatically generated from <tt>]m4_bpatsubst(b4_file_name, [^"\(.*\)"$], [\1])[</tt>.
  *
@@ -586,7 +586,6 @@ b4_define_state])[
     yystack.push (yystate, yylval ]b4_locations_if([, yylloc])[);
 ]m4_ifdef([b4_initial_action], [
 b4_dollar_pushdef([yylval], [], [yylloc])dnl
-    /* User initialization code.  */
     b4_user_initial_action
 b4_dollar_popdef[]dnl
 ])[
@@ -597,7 +596,6 @@ b4_dollar_popdef[]dnl
         push_parse_initialize ();
 ]m4_ifdef([b4_initial_action], [
 b4_dollar_pushdef([yylval], [], [yylloc])dnl
-    /* User initialization code.  */
     b4_user_initial_action
 b4_dollar_popdef[]dnl
 ])[

data/yacc.c

@@ -335,9 +335,7 @@ m4_if(b4_api_prefix, [yy], [],
 #define yychar          ]b4_prefix[char]b4_locations_if([[
 #define yylloc          ]b4_prefix[lloc]])]))[
 
-/* Copy the first part of user declarations.  */
 ]b4_user_pre_prologue[
-
 ]b4_null_define[
 
 /* Enabling verbose error messages.  */
@@ -354,9 +352,8 @@ m4_if(b4_api_prefix, [yy], [],
 ]])dnl
 b4_shared_declarations[
 
-/* Copy the second part of user declarations.  */
-]b4_user_post_prologue
-b4_percent_code_get[]dnl
+]b4_user_post_prologue[
+]b4_percent_code_get[]dnl
 
 [#ifdef short
 # undef short
@@ -1433,7 +1430,6 @@ b4_function_define([[yyparse]], [[int]], b4_parse_param)[
 ]m4_ifdef([b4_initial_action], [
 b4_dollar_pushdef([m4_define([b4_dollar_dollar_used])yylval], [],
                   [b4_push_if([b4_pure_if([*])yypushed_loc], [yylloc])])dnl
-/* User initialization code.  */
 b4_user_initial_action
 b4_dollar_popdef[]dnl
 m4_ifdef([b4_dollar_dollar_used],[[  yyvsp[0] = yylval;
@@ -1653,7 +1649,7 @@ yyreduce:
     int yychar_backup = yychar;
     switch (yyn)
       {
-        ]b4_user_actions[
+]b4_user_actions[
         default: break;
       }
     if (yychar_backup != yychar)