Jakefile: set __WIN32__ flag if node says plaform is win32
Platform.Var: removed

There is something wrong:
  If an IORef is sufficient then there is no need for promoting
  Var to an MVar on newer versions of GHC.
  If an IORef is not sufficient for the usecase it shouldn't be
  used in the first place.

I removed Platform.Var, replaced it with a direct reference to IOREf
and set a FIXME concerning the above issue.