fix add_bool_config in metaconfig.
From upstream's SVN commit 2634
required for librmagick-ruby 2.2.0

Index: ruby-setup/setup.rb
===================================================================
--- ruby-setup.orig/setup.rb	2011-05-30 13:27:58.171200339 +0200
+++ ruby-setup/setup.rb	2011-05-30 13:58:18.163201347 +0200
@@ -296,13 +296,14 @@
     ALIASES.each do |ali, name|
       @table[ali] = @table[name]
     end
-    @items.freeze
-    @table.freeze
-    @options_re = /\A--(#{@table.keys.join('|')})(?:=(.*))?\z/
+  end
+
+  def options_re
+    /\A--(#{@table.keys.join('|')})(?:=(.*))?\z/
   end
 
   def parse_opt(opt)
-    m = @options_re.match(opt) or setup_rb_error "config: unknown option #{opt}"
+    m = options_re().match(opt) or setup_rb_error "config: unknown option #{opt}"
     m.to_a[1,2]
   end