Bump macOS Xcode baseline to 11?

[Stephan Bergmann]

On 17/02/2020 14:48, Tor Lillqvist wrote:
> 
>     (For iOS, our baseline is documented as Xcode 9.3, too.  I have no
>     opinion whether or not we would want to bump that as well.
> 
> 
> Oh, definitely yes. IMHO we shouldn't even mention any specific version 
> but just tell people who want to build for iOS to use the latest Xcode.

I'm not going to touch that.  Whoever cares about the iOS build, please 
go ahead.