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.

TODO

@@ -27,12 +27,10 @@ Beware that returning 0 is unclear: does it mean there are no possible
 lookahead, or that there are too many?
 
 ** YYerror
-yacc.c should `#define YYERRCODE YYerror` in the *.c for sake of the
-projects that used it.  In particular
-
 https://git.savannah.gnu.org/gitweb/?p=gettext.git;a=blob;f=gettext-runtime/intl/plural.y;h=a712255af4f2f739c93336d4ff6556d932a426a5;hb=HEAD
 
-should be updated.
+should be updated to not use YYERRCODE.  Returning an undef token is good
+enough.
 
 ** Java
 *** Examples

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])

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