testsuite: Windows (7) does not reliably stop all threads when a process calls exit(). Update thread test to avoid this problem.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13669 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
testsuite: more cleanup of the makefiles

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13670 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02