Finally, after adding four packages and a huge build-fix patch, I've been able to build Webkit inside our cross-building infrastructure. That made Fancy buildable and allows for a better, more stable HTML rendering than with the old Gtkhtml2Viewer plugin. You can download and test it there: claws-mail-3.9.0cvs125pkg35.exe. Don't forget to unload GtkHTML2 and load Fancy...
