Clean DLLs in otherlibs/systhreads
Clean DLLs in otherlibs/systhreads

ocamlmklib produces bytecode dynamic link libraries for the threads
library but these are (intentionally) not installed.

The cleaning of them got lost in the merge of the two build systems.

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