brew/Library/Homebrew/requirements/minimum_macos_requirement.rb

require 'requirement'

class MinimumMacOSRequirement < Requirement
  fatal true

  def initialize(tags)
    @version = MacOS::Version.from_symbol(tags.first)
    super
  end

  satisfy(:build_env => false) { MacOS.version >= @version }

  def message
    "OS X #{@version.pretty_name} or newer is required."
  end
end