def path_to_apache_config_file(self):
        """Returns the full path to the apache configuration file.

        If the WEBKIT_HTTP_SERVER_CONF_PATH environment variable is set, its
        contents will be used instead.

        This is needed only by ports that use the apache_http_server module.
        """
        config_file_from_env = self.host.environ.get(
            'WEBKIT_HTTP_SERVER_CONF_PATH')
        if config_file_from_env:
            if not self._filesystem.exists(config_file_from_env):
                raise IOError(
                    '%s was not found on the system' % config_file_from_env)
            return config_file_from_env

        config_file_name = self._apache_config_file_name_for_platform()
        return self._filesystem.join(self.apache_config_directory(),
                                     config_file_name)