if think defining the constant is just papering over the real issue: the PHP LDAP extension is not present.
Does it make sense to display the settings page even if we know for sure that we can't use the plugin? (until we install and enable the PHP extension). Right now we do it in 1.9; Moodle 2.0 simply warns the user about the situation and doesn't try to show the rest of the settings page.
1.9 is also different in that we don't use the adminlib library to manage the enrol LDAP settings.This means we can't break install/upgrade if the constant is not defined. Unless you go to the enrol LDAP settings page, you won't notice at all (and only if you have Notices enabled in your PHP error level settings).
So I'd either leave it alone (nothing breaks, only an ungly notice is displayed) or do the same 2.0 does: don't try to display the settings if the extension is not available. But the latter is a change from the previous behaviour, in a stable branch. I'm not sure whether that's an acceptable change or not.