Re: Apple LLVM 10 and `__fallthrough__`

[Alexei Podtelezhnikov]

> > > -# elif (__GNUC__ >= 7) || (__clang_major__ >= 10)
> > > +# elif ((__GNUC__ >= 7) \
> > > +        || (defined __apple_build_version__ \
> > > +            ? __apple_build_version__ >= 14000000 \
> > > +            : __clang_major__ >= 10))
> > >
> > > Wiki suggests __apple_build_version__ >= 12000000
> > > https://en.wikipedia.org/wiki/Xcode#Xcode_11.0_-_14.x_(since_SwiftUI_framework)_2

Alternative Xcode versioning source found:
https://github.com/gsl-lite/gsl-lite/blob/master/include/gsl/gsl-lite.hpp#L534-L557