Predefined Versions for Apple Operating Systems

[Walter Bright]

I'm glad to see you use static assert to signal unsupported combinations, rather 
than falling into the trap of "what does the default do"!

There was an attempt a few years back to replace versions with `static 
if(Expression)`, as then version algebra could be used in druntime.

It got overly complicated and had chicken-and-egg problems.

I replaced it with versions.