In short, there is no quick/good way.
There are a few things you can do, first of all, set WITH_LDAP_MODULES in your port so that when apache gets pulled in it's built with LDAP.
People who already have Apache installed without LDAP will need to (manually) reinstalled apache, you will need to check if apache is installed with LDAP (For example by checking if a certain file exists, or by grepping apache --version) and bail out if apache isn't built with LDAP ... Yes, I know this sucks ...