warning - base incorrectly sets SDK version with macOS 11.1 / Xcode 12.3

[Joshua Root]

On 2020-12-16 02:53 , Christopher Jones wrote:
> 
> Suggestion for a fix...
> 
> <https://github.com/macports/macports-base/pull/221>

I don't think we can assume that users will always update the OS and
Xcode in lockstep.

- Josh