ocamlbuild: better provenance for syntax error messages -- full

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15117 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6087 ocamlbuild: improve _tags parsing of escaped newlines

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15118 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02