yacc.c: install backward compatibility for YYERRCODE

Some people have been using that symbol.  Some even have #defined it
themselves.
https://lists.gnu.org/r/bison-patches/2020-04/msg00138.html

Let's provide backward compatibility, having it point to YYUNDEF, so
that an error message is generated.

* data/skeletons/yacc.c (YYERRCODE): New, at the exact same location
it was defined before.

data/skeletons/yacc.c

@@ -746,6 +746,9 @@ enum { YYENOMEM = -2 };
       }                                                           \
   while (0)
 
+/* Backward compatibility with an undocumented macro.
+   Use ]b4_symbol(1, id)[ or ]b4_symbol(2, id)[. */
+#define YYERRCODE ]b4_symbol(2, id)[
 ]b4_locations_if([[
 ]b4_yylloc_default_define[
 #define YYRHSLOC(Rhs, K) ((Rhs)[K])