title: Macro `__unix__` not defined in MacOS X
body: <p>I noticed that in MacOS X (Lion), the macro <code>__unix__</code> is not defined. 
  Since MacOS has its roots in BSD UNIX, shouldn't that be the case?</p>
  
  <p>Is it possible to let the compiler/preprocessor know that I want it to be considered a UNIX system?</p>