build: fix warnings (shown on IRIX)

Appearing on IRIX with gcc -mabi=n32.
Reported by Bruno Haible.
https://lists.gnu.org/r/bug-bison/2020-05/msg00039.html

* examples/c++/variant-11.yy, examples/c/bistromathic/parse.y: Don't
give chars to isdigit, cast them to unsigned char before.
* src/complain.c: Use c_isdigit.
* src/fixits.c (fixits_run): Avoid casts.
* src/lalr.c (goto_print): Use %zu for a size_t.

examples/c/bistromathic/parse.y

@@ -363,7 +363,7 @@ yyreport_syntax_error (const yypcontext_t *ctx)
       }
     // %0e, %1e...: expected token.
     else if (format[0] == '%'
-             && isdigit (format[1])
+             && isdigit ((unsigned char) format[1])
              && format[2] == 'e'
              && (format[1] - '0') < argsize)
       {