Add console os constraints and config_settings.

[zjturner]

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[IanChilds]

Why do these need to go into prelude? Where are they going to be used?

[zjturner]

I am not sure where else it could really go and be usable. It would be used anywhere that a config//os: constraint is currently used. There's too many to list, but just as one example, this is a frequent use case for us:

cxx_library(
    name = "foo",
    srcs = [
        <common sources>
    ] + select({
        "config//os:windows": _windows_sources,
        "config//os:linux": _linux_sources,
        "config//os:macos": _macos_sources,
        "config//os:ps4": _playstation_sources,   # we need this
    })
)

Same argument applies to deps, and most other selectable args.

When deciding whether it made sense to put these here, I looked at the surrounding code in the file. There are other constraints like illumos, netbsd, and fuchsia that if you grep the prelude for them, 0 hits come up. And these are prefaced by a comment that says "Long tail but contemporarily relevant operating systems". I assume Meta internally has lots of internal build code that refers to these OSes, and the same would apply here. So it seems appropriate to me for these to be here, but LMK your thoughts.