code: def constant(self, name, value):
          
          assert is_iterable_typed(name, basestring)
          assert is_iterable_typed(value, basestring)
          self.registry.current().add_constant(name[0], value)
docs: Declare and set a project global constant.
          Project global constants are normal variables but should
          not be changed. They are applied to every child Jamfile.