civimail.cronjob.php fails to honor drupal settings.php's $db_prefix

in drupal's sites/default/settings.php,
we set $db_prefix = 'd6_', our db name is 'friends'

when running wget -O - -q -t 1 http://.../sites/all/modules/civicrm/bin/civimail.cronjob.php
we get this error

[debug_info] = SELECT u.* FROM users u WHERE LOWER(u.name) = '...' AND u.pass = '...' AND u.status = 1 [nativecode=1146 ** Table 'friends.users' doesn't exist]

but we expect the query should have been

SELECT u.* FROM d6_users ...

improperly worked around this in CRM/Utils/System/Drupal.php's authenticate()

(had tried to set userFrameworkUsersTableName in CRM/Core/Config.php's _setUserFrameworkConfig()
but this didnt seem to work)