Cyrus-SASL scans the PLugin-Directory (/usr/lib/sasl2/) for it's libs. It is senseless to rebuild Cyrus-SASL without one Plugin if it is already in the Directory.
I realized that after I replied to your message. Thanks for the hint!
Now SASL/DIGEST-MD5 mechanism is selected by default, but it's not authorizing anyone, not even root.