Add a new alternative for this target.

def add_alternative (self, target):
        """ Add a new alternative for this target.
        """
        assert isinstance(target, BasicTarget)
        d = target.default_build ()

        if self.alternatives_ and self.default_build_ != d:
            get_manager().errors()("default build must be identical in all alternatives\n"
              "main target is '%s'\n"
              "with '%s'\n"
              "differing from previous default build: '%s'" % (self.full_name (), d.raw (), self.default_build_.raw ()))

        else:
            self.default_build_ = d

        self.alternatives_.append (target)