title: What's with "#ifdef _MAC" in Windows header files?
body: <p>I was browsing through Windows's Platform SDK header files (what a life, right?), and I noticed many places contained references to the preprocessor symbol <code>_MAC</code>. For example:</p>
  
  <pre><code>// WinUser.h line 1568
  /*
   * Message structure
   */
  typedef struct tagMSG {
      HWND        hwnd;
      UINT        message;
      WPARAM      wParam;
      LPARAM      lParam;
      DWORD       time;
      POINT       pt;
  #ifdef _MAC
      DWORD       lPrivate;
  #endif
  } MSG, *PMSG, NEAR *NPMSG, FAR *LPMSG;
  </code></pre>
  
  <p>Does this mean "Macintosh", as it appears? Was there a time where Windows or a subset of Windows could be compiled for the Macintosh?</p>