Add minimum OS X version requirement

Closes Homebrew/homebrew#19998.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/dependency_collector.rb

@@ -87,6 +87,7 @@ class DependencyCollector
       # so just return a standard formula dependency.
       Dependency.new(spec.to_s, tags)
     when :xcode      then XcodeDependency.new(tags)
+    when :macos      then MinimumMacOSRequirement.new(tags)
     when :mysql      then MysqlDependency.new(tags)
     when :postgresql then PostgresqlDependency.new(tags)
     when :tex        then TeXDependency.new(tags)

Library/Homebrew/requirements.rb

@@ -3,6 +3,7 @@ require 'requirements/language_module_dependency'
 require 'requirements/x11_dependency'
 require 'requirements/mpi_dependency'
 require 'requirements/python_dependency'
+require 'requirements/macos_requirement'
 
 class XcodeDependency < Requirement
   fatal true

Library/Homebrew/requirements/macos_requirement.rb

@@ -0,0 +1,16 @@
+require 'requirement'
+
+class MinimumMacOSRequirement < Requirement
+  fatal true
+
+  def initialize(tags)
+    @version = MacOS::Version.from_symbol(tags.first)
+    super
+  end
+
+  satisfy { MacOS.version >= @version }
+
+  def message
+    "OS X #{@version.pretty_name} or newer is required."
+  end
+end