I managed to fix the error by using fully qualified names on the modules (wasn't sure if this was necessary as they were both in the same sub-folder)
That led me to another error along the lines of & ...
I managed to build gtkD successfully on one XP computer, but a couple of hours later I was going to do the same on my home computer (also XP), and bud_win.exe (dub/build) gives me the follow ...