sets up a 'legacy mode'
When the OCAML_LEGACY_FORMAT environment variable is set, the compiler
will try to emulate the behaviour of the previous implementation of
format.
git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14813 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02