title: inbuilt std::__gcd() function for c++ is not working on Mac OS Xcode
body: <p>Inbuilt __gcd() function is not working on Xcode macOS.
  I have run the following code on Xcode (macOS Catalina) it shows an error
  "Use of undeclared identifier '__gcd' ".</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;algorithm&gt;
  
  using namespace std;
  
  int main()
  {
      cout &lt;&lt; "gcd(6, 20) = " &lt;&lt; __gcd(6, 20) &lt;&lt; endl;
      return 0;
  
  }
  </code></pre>
  
  <p>please help me out</p>