Define MacOS.version, MacOS.full_version, and MacOS::Xcode.version to Version::NULL on Linux so that brew readall succeeds and Homebrew/brew can tap Homebrew/core on Linux.
Library
This directory contains all the code run by the official brew and brew cask commands in Homebrew and all formulae (package descriptions) in taps (repositories containing formulae) in Taps subdirectories.