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.

def constant(self, name, value):
        """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."""
        assert is_iterable_typed(name, basestring)
        assert is_iterable_typed(value, basestring)
        self.registry.current().add_constant(name[0], value)