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.

src/complain.c

@@ -23,7 +23,7 @@
 #include "system.h"
 
 #include <argmatch.h>
-#include <ctype.h>
+#include <c-ctype.h>
 #include <progname.h>
 #include <stdarg.h>
 #include <sys/stat.h>
@@ -609,7 +609,7 @@ syntax_error (location loc,
 
   while (*format)
     if (format[0] == '%'
-        && isdigit (format[1])
+        && c_isdigit (format[1])
         && format[2] == '$'
         && format[3] == 's'
         && (format[1] - '0') < argc)