Reinstate installation of dllthreads$(EXT_DLL)
Reinstate installation of dllthreads$(EXT_DLL)

It was accidently dropped by 5c4c41ba

msprotz/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,msprotz/ocaml