Support assignment from 0, construction from 0, and comparison to zero.

[SVN r16174]

include/boost/function/detail/prologue.hpp

@@ -6,6 +6,7 @@
 #  include <boost/config.hpp>
 #  include <boost/function/function_base.hpp>
 #  include <boost/mem_fn.hpp>
+#  include <boost/type_traits/is_same.hpp>
 #  include <boost/preprocessor/enum.hpp>
 #  include <boost/preprocessor/enum_params.hpp>
 #  include <boost/preprocessor/cat.hpp>