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.

src/parse-gram.c

@@ -931,6 +931,9 @@ enum { YYENOMEM = -2 };
       }                                                           \
   while (0)
 
+/* Backward compatibility with an undocumented macro.
+   Use GRAM_error or GRAM_UNDEF. */
+#define YYERRCODE GRAM_UNDEF
 
 /* YYLLOC_DEFAULT -- Set CURRENT to span from RHS[1] to RHS[N].
    If N is 0, then set CURRENT to the empty location which ends