Okay, managed to sort this one.
With the openldap, openldap-devel and openldap-clients packages installed (I don't know how necassary they each are), I added '--with-ldap \' to the bottom of the '--with' part of configure.php in /usr/local/directadmin/customapache.
Still in the customapache directory, I then typed './build clean', then './build php n'. Restarted httpd, and all seemed fine!