Backends/gtkairo/ffi.lisp has a defcfun entry for XGetErrorText which as far as I can tell is not exported by any of the Win32 GTK DLLs (that makes sense, although one could imagine it being defined for interoperability reasons). The dribble-x-errors function in Backends/gtkairo/event.lisp uses a conditional to avoid calling XGetErrorText on Win32. I've changed my local copy of ffi.lisp to avoid the defcfun with the same conditional; perhaps the same fix could be checked in?
Thanks.