Revert "PR#6270: remove need for -I directives to ocamldebug in common case (patch by Josh Watzman)"

There seems to be a problematic interaction with backtrace collection. To test this,
  make world.opt
  cd testsuite
  make one DIR=tests/backtrace/

This needs to be sorted out before the patch can go in again.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14510 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02