harmonize Makefile.nt wrt. Makefile to simplify future line-by-line comparison

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14535 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6358: obey DESTDIR in install targets

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14536 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02