macos: add fallback version detection

[mikdusan]

Fallback to sysctl kern.osversion when kern.osproductversion is not
available (prior to 10.13.4) .

The mapping from sw_vers -buildVersion to -productVersion is
formulaic from 10.8 to 10.15 and older is handled with switch.

closes#5119

The reasoning behind using a table is we do not yet have std XML to parse /System/Library/CoreServices/SystemVersion.plist and writing a non-robust XML parser has unknown risks; old copies of SystemVersion.plist are hard to come by.

That said, at this time I expect this table fallback will suffice until we get XML. At that time I would recommend we switch methods because it's actually via that file at startup that the kernel gets its product version.

Also I'd like to add this table is based on mapping kern.osversion to kern.osproductversion. To use kern.osrelease (Darwin version) would be difficult -- either the Darwin version tables on wikipedia are incomplete or it does not bump when kern.osproductversion does. For example, Darwin 19.0.0 is inferred to be both macOS 10.15.0 and 10.15.1. source

[daurnimator]

Got a reference link for the build number mapping?

[mikdusan]

Got a reference link for the build number mapping?

yes added with last push

[andrewrk]

This looks useful, nice work.

Now that there is a macos-specific non-trivial data listing here, would you mind moving the macos implementation of this function into a macos-specific separate file? For example lib/std/zig/system/macos.zig

[mikdusan]

Dear Apple: just because you can, doesn't mean you should™️

Having finally upgraded to Big Sur and look at what happens to a file.. yes a file! This means we can no longer parse /System/Library/CoreServices/SystemVersion.plist as originally planned:

*take note same file being cat'd twice with different results just because of environment variable which does the same thing as if the binary (in this case cat) were built on an pre-Big Sur SDK.