Module should warn if LDAP support is not available in PHP

When you go to admin/settings/ldapauth you should see a clear warning message if the LDAP support is not available in PHP.
It should be simply something like:

if (!function_exists('ldap_connect'))
drupal_set_message(t('LDAP support is not available in PHP! Please enable it.'), 'error');

(just written the code here, not tested).