`Select()` on deployment target (aka minimum_os_version)

Hey @keith and others,

Is there currently a good way to `select()` on deployment target (or minimum_os_version, as the attribute is named in the rules) as an inequality?
If not, I'd love to ask for it to be considered as a new feature.
(Asking only because I couldn't find, searching around. I can imagine one might be able to select on the command line flag, but I'm not sure how you'd implement the inequality portion--perhaps it would need an exhaustive list?)

[In case it helps, the particular use case I'm seeing right now is needing to fall back to boost::filesystem (& link) if the deployment target is less than iOS 13 or macOS 10.15.]

Thanks for all you do,
Chris

Start by reading the Apple rules' handling of the minimum_os_version deployment-target attribute and any existing select() configuration paths, including command-line selection. Determine whether inequality matching can be supported for iOS 13 and macOS 10.15 targets, and consider the implications of an exhaustive version list; done means a decided, tested feature scope.

Written by the indexing model from the issue text.