guess the value for --with-pam-seclib-dir if it is not specified (0b5b4d1 from 0.9)

git-svn-id: http://arthurdejong.org/svn/nss-pam-ldapd/nss-pam-ldapd-0.8@1953 ef36b2f9-881f-0410-afb5-c4e39611909c