Add missing ldap_raname_s() function availability check

If ldap_add_ext_s() is found, the current code assumes that
ldap_rename_s() is also found. But it is not correct. We should check
ldap_rename_s() function availability for ldap_rename_s().
Merge pull request #24 from kou/add-missing-rename-check

Add missing ldap_raname_s() function availability check