rhash: add Lion requirement

Closes Homebrew/homebrew#17132.

Formula/rhash.rb

@@ -1,10 +1,24 @@
 require 'formula'
 
+class LionOrNewer < Requirement
+  fatal true
+
+  def satisfied?
+    MacOS.version >= :lion
+  end
+  def message
+    "rhash requires `wcsdup` which isn't in the SDK before Lion."
+  end
+end
+
+
 class Rhash < Formula
   homepage 'http://rhash.anz.ru/'
   url 'http://downloads.sourceforge.net/project/rhash/rhash/1.2.10/rhash-1.2.10-src.tar.gz'
   sha1 '130f55faf3f13760ef0ab6a25e52db5052064c63'
 
+  depends_on LionOrNewer.new
+
   def install
     system 'make', 'install', "PREFIX=",
                               "DESTDIR=#{prefix}",