use older stat tools. TurboC doesn't know what S_ISDIR is

.gitignore

@@ -2,4 +2,5 @@
 .grok/
 .idea/
 .vscode/
+.DS_Store
 build/

src/cnadd.c

@@ -11,8 +11,8 @@ int ensure_directory_exists(const char *path) {
     struct stat st;
     
     if (stat(path, &st) == 0) {
-        /* Path exists, check if it's a directory */
-        if (S_ISDIR(st.st_mode)) {
+        /* Path exists, check if it's a directory using bit test */
+        if ((st.st_mode & S_IFMT) == S_IFDIR) {
             return 1;
         } else {
             fprintf(stderr, "Error: %s exists but is not a directory\n", path);