You do, that's the point here. You need ifdefs in this example to selectively compile the same code with different compilers, or different language levels with the same compiler. In either case C3 will fail while parsing a newer feature with an older compiler or lang level.
This exact situation is there in Java, using compiler args, after lambda functions were introduced. Older compilers would not be able to handle it, and the newer compilers would not break backward compatibility.