windows: don't define lseeki64 and lseek since they already exist.

Patch by Adrien Nader!

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13840 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
configure: use inf/wrn/err functions to write messages for the user.
(Patch by Adrien Nader!)

This makes messages to the user more consistent.

Behaviours:
  inf(): writes its arguments to stderr
  wrn(): writes "[WARNING]" to stderr and then its arguments to stderr
  err(): writes "[ERROR!]" to stderr, then its arguments to stderr, and
         calls "exit 2"

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13841 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02