m4: catch suspicions of unevaluated macros

Check in m4's output if there are sequences such as m4_foo or b4_foo,
which are probably resulting from incorrect m4 processing.

It actually already is useful:

- it caught a leaking b4_lac_if leaking from glr.c, where LAC is not
  supported, hence b4_lac_if is not defined.

- it also caught references to location.hh in position.hh when
  location.hh does not exist.

- while making "Code injection" robust to these new warnings (it is
  its very purpose to let b4_canary pass unevaluated), I saw that it
  did not check lalr1.d, and when adding lalr1.d, it revealed it did
  underquote ocurrences of token value types.

* src/scan-skel.l (macro): New abbreviation.
Use it.
* data/skeletons/glr.c: Don't use b4_lac_if, we don't have it.
* data/skeletons/location.cc: Don't generate position.hh when we don't
generate location.hh.
* data/skeletons/d.m4 (b4_basic_symbol_constructor_define): Fix
underquotation.
* data/skeletons/bison.m4 (b4_canary): New.
* tests/input.at (Code injection): Use it, and check lalr1.d too.

data/skeletons/bison.m4

@@ -261,6 +261,13 @@ m4_define([b4_fatal_at],
 [b4_error([[fatal]], $@)dnl
 m4_exit(1)])
 
+# b4_canary(MSG)
+# --------------
+# Issue a warning on stderr and in the output.  Used in the test suite
+# to catch spurious m4 evaluations.
+m4_define([b4_canary],
+[m4_errprintn([dead canary: $1])DEAD CANARY($1)])
+
 
 ## ------------ ##
 ## Data Types.  ##

data/skeletons/d.m4

@@ -586,7 +586,7 @@ alias ACCEPT = ]b4_parser_class[.YYACCEPT;]])[]])[
 # Create Symbol struct constructors for all the visible types.
 m4_define([b4_basic_symbol_constructor_define],
 [b4_token_visible_if([$1],
-[    this(TokenKind token]b4_symbol_if([$1], [has_type],
+[[    this(TokenKind token]b4_symbol_if([$1], [has_type],
 [[, ]b4_union_if([], [[typeof(YYSemanticType.]])b4_symbol([$1], [type])dnl
 []b4_union_if([], [[) ]])[ val]])[]b4_locations_if([[, Location loc]])[)
     {
@@ -601,7 +601,7 @@ m4_define([b4_basic_symbol_constructor_define],
       value_.]b4_symbol([$1], [type])[ = val;]])])[]b4_locations_if([
       location_ = loc;])[
     }
-])])
+]])])
 
 
 # b4_symbol_type_define

data/skeletons/glr.c

@@ -2204,12 +2204,7 @@ yy_syntax_error_arguments (const yyGLRStack* yystackp,
      - Don't assume there isn't a lookahead just because this state is a
        consistent state with a default action.  There might have been a
        previous inconsistent state, consistent state with a non-default
-       action, or user semantic action that manipulated yychar.]b4_lac_if([[
-       In the first two cases, it might appear that the current syntax
-       error should have been detected in the previous state when yy_lac
-       was invoked.  However, at that time, there might have been a
-       different syntax error that discarded a different initial context
-       during error recovery, leaving behind the current lookahead.]], [[
+       action, or user semantic action that manipulated yychar.
      - Of course, the expected token list depends on states to have
        correct lookahead information, and it depends on the parser not
        to perform extra reductions after fetching a lookahead from the
@@ -2217,7 +2212,7 @@ yy_syntax_error_arguments (const yyGLRStack* yystackp,
        (from LALR or IELR) and default reductions corrupt the expected
        token list.  However, the list is correct for canonical LR with
        one exception: it will still contain any token that will not be
-       accepted due to an error action in a later state.]])[
+       accepted due to an error action in a later state.
   */
   if (yytoken != ]b4_symbol(empty, kind)[)
     {

data/skeletons/location.cc

@@ -19,13 +19,6 @@ m4_pushdef([b4_copyright_years],
            [2002-2015, 2018-2021])
 
 
-# b4_position_file
-# ----------------
-# Name of the file containing the position class, if we want this file.
-b4_header_if([b4_required_version_if([30200], [],
-                                     [m4_define([b4_position_file], [position.hh])])])])
-
-
 # b4_location_file
 # ----------------
 # Name of the file containing the position/location class,
@@ -54,6 +47,16 @@ m4_ifdef([b4_location_file],
  ])
 
 
+# b4_position_file
+# ----------------
+# Name of the file containing the position class, if we want this file.
+b4_header_if(
+  [b4_required_version_if(
+    [30200], [],
+    [m4_ifdef([b4_location_file],
+              [m4_define([b4_position_file], [position.hh])])])])
+
+
 
 # b4_location_define
 # ------------------