c: don't emit an error message when the scanner returns YYERRCODE

* data/skeletons/yacc.c (yyparse): When the scanner returns YYERRCODE,
go directly to error recovery (yyerrlab1).
However, don't keep the error token as lookahead, that token is too
special.
* data/skeletons/lalr1.cc: Likewise.

* examples/c/bistromathic/parse.y (yylex): Use that feature to report
nicely invalid characters.
* examples/c/bistromathic/bistromathic.test: Check that.
* examples/test: Neutralize gratuitous differences such as rule
position.

* tests/calc.at: Check that case in C only.
The other case seem to be working, but that's an illusion that the
next commit will address (in fact, they can enter endless loops, and
report the error several times anyway).

examples/c/bistromathic/parse.y

@@ -3,6 +3,7 @@
 %code top {
   #include <ctype.h>  // isdigit
   #include <math.h>   // cos, sin, etc.
+  #include <stdarg.h> // va_start
   #include <stdio.h>  // printf
   #include <stdlib.h> // calloc
   #include <string.h> // strcmp
@@ -45,8 +46,14 @@
 }
 
 %code provides {
+# ifndef __attribute__
+#  ifndef __GNUC__
+#   define __attribute__(Spec) /* empty */
+#  endif
+# endif
   int yylex (const char **line, YYSTYPE *yylval, YYLTYPE *yylloc);
-  void yyerror (YYLTYPE *yylloc, char const *msg);
+  void yyerror (YYLTYPE *loc, char const *format, ...)
+    __attribute__ ((__format__ (__printf__, 2, 3)));
 }
 
 %code {
@@ -249,6 +256,8 @@ yylex (const char **line, YYSTYPE *yylval, YYLTYPE *yylloc)
     case '(': return TOK_LPAREN;
     case ')': return TOK_RPAREN;
 
+    case '!': return TOK_YYUNDEF;
+
     case '\0': return TOK_YYEOF;
 
       // Numbers.
@@ -290,8 +299,8 @@ yylex (const char **line, YYSTYPE *yylval, YYLTYPE *yylloc)
 
       // Stray characters.
     default:
-      yyerror (yylloc, "error: invalid character");
-      return yylex (line, yylval, yylloc);
+      yyerror (yylloc, "syntax error: invalid character: %c", c);
+      return TOK_YYERRCODE;
     }
 }
 
@@ -371,10 +380,15 @@ yyreport_syntax_error (const yypcontext_t *ctx)
 
 
 // Called by yyparse on error.
-void yyerror (YYLTYPE *loc, char const *msg)
+void yyerror (YYLTYPE *loc, char const *format, ...)
 {
   YY_LOCATION_PRINT (stderr, *loc);
-  fprintf (stderr, ": %s\n", msg);
+  fputs (": ", stderr);
+  va_list args;
+  va_start (args, format);
+  vfprintf (stderr, format, args);
+  va_end (args);
+  putc ('\n', stderr);
 }
 
 
@@ -535,7 +549,7 @@ int main (int argc, char const* argv[])
 #endif
 
   // Enable parse traces on option -p.
-  if (argc == 2 && strcmp (argv[1], "-p") == 0)
+  if (1 < argc && strcmp (argv[1], "-p") == 0)
     yydebug = 1;
   init_table ();
   init_readline ();