Seems to be impossible to compile the plugin with the sources of Xfce 4.8. Due to the API change, the file xfce-panel-window.h couldn't be found: wmdock.c:38:45: fatal error: libxfce4panel/xfce-panel-window.h: No such file or directory compilation terminated. I tried to build a Fedora package. The scratch build on the Koji build server is here: http://koji.fedoraproject.org/koji/taskinfo?taskID=2950793
Should be fixed.