- Use a more conformant C++ preprocessor macro identifier for Unix systems.

The 'unix' identifier isn't defined when '-std' is passed to the compiler (tested with gcc and clang), so use '__unix__' which is well enough documented.

src/menu/menudef.cpp

@@ -158,7 +158,7 @@ static bool CheckSkipOptionBlock(FScanner &sc)
 		}
 		else if (sc.Compare("unix"))
 		{
-			#ifdef unix
+			#ifdef __unix__
 				filter = true;
 			#endif
 		}