MacOSX11.sdk

[Ryan Schmidt]

I notice that the Xcode 12.5 CLT (but not Xcode 12.5) now contains a 
MacOSX11.sdk symlink pointing to the MacOSX11.3.sdk. If MacPorts used 
MacOSX11.sdk when available instead of the more-specific version number, it 
would reduce some of the problems we have from baked-in SDK paths in some 
ports. Would be nice if we could fit that change into the MacPorts 2.7.0 
release.