Mono에서 Platform OS 구분하기

http://stackoverflow.com/questions/9129491/c-sharp-compiled-in-mono-detect-os

string msg1 = "This is a Windows operating system.";
string msg2 = "This is a Unix operating system.";
string msg3 = "ERROR: This platform identifier is invalid.";

OperatingSystem os = Environment.OSVersion;
PlatformID     pid = os.Platform;
switch (pid) 
    {
    case PlatformID.Win32NT:
    case PlatformID.Win32S:
    case PlatformID.Win32Windows:
    case PlatformID.WinCE:
        Console.WriteLine(msg1);
        break;
    case PlatformID.Unix:
        Console.WriteLine(msg2);
        break;
    default:
        Console.WriteLine(msg3);
        break;
    }
}

자~ 그럼 Compile 타임에 이걸 알 수 있는 방법은?