Collation version tracking for macOS

During development, I have been using the attached patch to simulate 
libc collation versions on macOS.  It just uses the internal major OS 
version number.  I don't know to what the extend the libc locales on 
macOS are maintained or updated at all, so I don't know what practical 
effect this would have.  Again, it's mainly for development.  If there 
is interest from others, I think we could add this, maybe disabled by 
default, or we just keep it in the mailing list archives for interested 
parties.