Check if a build option is enabled.

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

        Example:

            {% if with('foo') %}...{% endif %}

def _with(self, option=None):
        """Check if a build option is enabled.

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

        Example:

            {% if with('foo') %}...{% endif %}
        """
        if option is None:
            return (
                (k, v) for k, v in self.env.items()
                if k.startswith('WITH_')
            )
        return self.env.get('WITH_' + option.upper()) == True