The original cyrusauth file modified to create this one was retrieved using apt-get source znc on Debian "Jessie". The changes should work if copied as-is to the source available on github or elsewhere, as is relevant to your system.
However, for cyrusauthldap to work, cyrusauthldapLDFLAGS := -lsasl2 has to be
added to the Makefile in modules/ in the souce. Try cat Makefile | grep cyrusauth to see that this has been done for cyrusauth as well.
Note, you may need to run ./configure --enable-cyrus for this to work.
Similarly, you most likely need to have installed znc with cyrusauth enabled,
which has been the default since 1.0.
Once it's compiled and the .so file put in the currently installed ZNC's
modules/ directory, connect to znc and /msg *status loadmod cyrusauthldap saslauthd, then set CreateUser to yes, set up CloneUser with a base user and
set CreateFromLDAP to yes.