macos_version: add KERNEL_MAJOR_VERSIONS map

[carlocab]

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This will be used by llvm (and, presumably, in the future, versioned
LLVM formulae). The idea is that we will write a config file for each OS
version pointing to the correct SDKROOT so that llvm does not require
rebuilding/reinstalling when a user upgrades to a new major version of
macOS.

See Homebrew/homebrew-core#196094.

[Bo98]

Looks good but maybe we probably should expose the API as a function wrapping this constant as it's easier to deprecate etc.

We can probably make it calculated to be honest:

if version.major == 10
  version.minor + 4
else
  version.major + 9
end

[MikeMcQuaid]

Good idea, thanks @carlocab!

[carlocab]

Looks good but maybe we probably should expose the API as a function wrapping this constant as it's easier to deprecate etc.

We can probably make it calculated to be honest:

if version.major == 10
  version.minor + 4
else
  version.major + 9
end

Cool, done in 42dd0ac.

[MikeMcQuaid]

Thanks @carlocab!