- 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/c_console.cpp

@@ -1742,7 +1742,7 @@ static bool C_HandleKey (event_t *ev, BYTE *buffer, int len)
 		}
 		break;
 
-#ifdef unix
+#ifdef __unix__
 	case EV_GUI_MButtonDown:
 		C_PasteText(I_GetFromClipboard(true), buffer, len);
 		break;