ocamldbi: add missing dependencies and conflicts

The Makefile for ocamldbi tests for the following libraries and compiles differently when they are present:
- postgres
- mysql
  --> I have added these two as optional dependencies
- postgresql
- freetds
  --> there are compilation errors when these are present, so I added them as conflicts
- perl4caml
- ocamlodbc
- sqlite
  --> these don't seem to be available on OPAM, so I left them alone