warnings: enable -Wuseless-cast, and eliminate warnings

Prompted by Frank Heckenbach.
https://lists.gnu.org/archive/html/bug-bison/2019-11/msg00016.html.

* configure.ac (warn_cxx): Add -Wuseless-cast.
* data/skeletons/c.m4 (b4_attribute_define): Define
YY_IGNORE_USELESS_CAST_BEGIN and YY_IGNORE_USELESS_CAST_END.
* data/skeletons/glr.c (YY_FPRINTF): New, replaces YYFPRINTF, wrapped
with YY_IGNORE_USELESS_CAST_BEGIN and YY_IGNORE_USELESS_CAST_END.
(YY_DPRINTF): Likewise.
* tests/actions.at: Remove useless cast.
* tests/headers.at: Adjust.

tests/actions.at

@@ -1780,7 +1780,7 @@ float: UNTYPED INT
                                  yy::parser::token::INT,
                                   EOF}]],
                                [[{UNTYPED, INT, EOF}]]),
-                 [AT_VAL.ival = YY_CAST (int, toknum) * 10;
+                 [AT_VAL.ival = toknum * 10;
                   AT_VAL.fval = YY_CAST (float, toknum) / 10.0f;])[
 ]AT_MAIN_DEFINE[
 ]])
@@ -1897,7 +1897,7 @@ exp:
 
 %%
 ]AT_YYERROR_DEFINE[
-]AT_YYLEX_DEFINE(["bcd"], [*lvalp = YY_CAST (int, (toknum + 1) * 10)])[
+]AT_YYLEX_DEFINE(["bcd"], [*lvalp = (toknum + 1) * 10])[
 ]AT_MAIN_DEFINE[
 ]])
 AT_BISON_OPTION_POPDEFS

tests/headers.at

@@ -326,7 +326,7 @@ AT_PERL_CHECK([[-n -0777 -e '
       |YY_CONSTEXPR
       |YY_COPY
       |YY_CPLUSPLUS
-      |YY_IGNORE_MAYBE_UNINITIALIZED_(?:BEGIN|END)
+      |YY_IGNORE_(?:MAYBE_UNINITIALIZED|USELESS_CAST)_(?:BEGIN|END)
       |YY_INITIAL_VALUE
       |YY_MOVE
       |YY_MOVE_OR_COPY