As an alternative you can use:
string DXSDK_DIR = Environment.GetEnvironmentVariable("DXSDK_DIR", EnvironmentVariableTarget.Machine); // use EnvironmentVariableTarget.User if you added it on the user side instead of the system side.
PublicLibraryPaths.Add(DXSDK_DIR + Path.DirectorySeparatorChar + "Lib" + Path.DirectorySeparatorChar + LibFolder);