use older stat tools for dos; newer stat tools for linux

src/cnadd.c

@@ -11,10 +11,19 @@ int ensure_directory_exists(const char *path) {
     struct stat st;
     
     if (stat(path, &st) == 0) {
-        /* Path exists, check if it's a directory using bit test */
+        /* Path exists, check if it's a directory */
+#if defined(__MSDOS__) || defined(__DOS__)
+        /* DOS/Turbo C: use bit test because S_ISDIR macro may be missing */
         if ((st.st_mode & S_IFMT) == S_IFDIR) {
             return 1;
-        } else {
+        }
+#else
+        /* Unix/Linux/Windows modern: use standard POSIX macro */
+        if (S_ISDIR(st.st_mode)) {
+            return 1;
+        }
+#endif
+        else {
             fprintf(stderr, "Error: %s exists but is not a directory\n", path);
             return 0;
         }