Add OS.mac? and OS.linux?

Library/Homebrew/extend/ENV/std.rb

Library/Homebrew/formula_installer.rb

Library/Homebrew/global.rb

Library/Homebrew/hardware.rb

Library/Homebrew/keg_fix_install_names.rb

Library/Homebrew/macos.rb

Library/Homebrew/os.rb

Library/Homebrew/os/mac/xcode.rb

Library/Homebrew/test/testing_env.rb

Library/brew.rb