Check if a configure flag is set.

        If called without argument, it returns all HAVE_* items.

        Example:

            {% if have('netinet/ip.h') %}...{% endif %}

def _have(self, name=None):
        """Check if a configure flag is set.

        If called without argument, it returns all HAVE_* items.

        Example:

            {% if have('netinet/ip.h') %}...{% endif %}
        """
        if name is None:
            return (
                (k, v) for k, v  in self.env.items()
                if k.startswith('HAVE_')
            )
        return self.env.get('HAVE_' + self.env_key(name)) == True