Message encryption is not supported on Mac

[nitzmahone]

The Heimdal Kerberos version that ships with MacOS does not appear to include the necessary bits to support Kerberos message encryption. Some of the Apple headers in GSS.Framework include reference to "Apple Private SPI" headers that do include what's necessary, but they're not shipped to users, are are thus unavailable to build against.

If we're able to get the necessary stuff to build locally on a Mac, we might explore releasing a bdist_wheel that includes message encryption, but we're not even sure how many people would want to do it.