Use a mutex to make SortedSet.setup thread-safe

This should fix [Bug #13735].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60304 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/set.rb

@@ -634,6 +634,7 @@ end
 #
 class SortedSet < Set
   @@setup = false
+  @@mutex = Mutex.new
 
   class << self
     def [](*ary)        # :nodoc:
@@ -643,6 +644,7 @@ class SortedSet < Set
     def setup   # :nodoc:
       @@setup and return
 
+      @@mutex.synchronize do
         # a hack to shut up warning
         alias_method :old_init, :initialize
 
@@ -729,13 +731,13 @@ class SortedSet < Set
             end
           END
         end
-
         # a hack to shut up warning
         remove_method :old_init
 
         @@setup = true
       end
     end
+  end
 
   def initialize(*args, &block) # :nodoc:
     SortedSet.setup