The development of xdkim was stopped. The project was incorporated into the successor  qdkim 

xdkim was forked from libdkim. It is a least like the tool  libdkimtest  with some improvements. The library functions are linked statically. See CHANGELOG for details. Thus xdkim does what it should do. It is preferred to be used with the qmail-xdim package.

The buffer overflow mentioned by Joshua Rogers is fixed in xdkim.

Download from GitHub: xdkim releases (no longer available!)

xdkim qdkim will be discussed on the openqmail mailing list.

Have a look at the README file in the package to see how to install it.