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.

src/scan-skel.l

@@ -53,6 +53,9 @@ static void fail_for_invalid_at (char const *at);
 static void output_mapped_file (char const *name);
 %}
 
+/* Identifiers of our M4 macros.  */
+macro    [bm]4_[a-zA-Z_0-9]*
+
 %x SC_AT_DIRECTIVE_ARGS
 %x SC_AT_DIRECTIVE_SKIP_WS
 
@@ -87,7 +90,21 @@ static void output_mapped_file (char const *name);
   /* This pattern must not match more than the previous @ patterns. */
 @[^@{}''(\n]*   fail_for_invalid_at (yytext);
 \n              out_lineno++; ECHO;
-[^@\n]+         ECHO;
+[^bm@\n]+       ECHO;
+
+  /* If there are still identifiers that look like macros, such as
+     b4_synbol, this probably an error, say a typo in M4, or
+     overquotation.  */
+{macro}         {
+                  location loc = empty_loc;
+                  loc.start.file = map_file_name (out_name);
+                  loc.start.line = out_lineno;
+                  loc.end = loc.start;
+                  complain (&loc, Wother,
+                            "suspicious sequence in the output: %s", yytext);
+                  ECHO;
+                }
+.               ECHO;
 
 <INITIAL><<EOF>> {
   if (out_name)