Installation fails when `LIBDIR` and `OCAMLINSTALLLIB` are the same directory
clean_old_install-lib rules erase too many files when
OCAMLINSTALLLIB happen to be the same directory. This is the case on Opam for example. As a consequence, doing
make install-lib erases all the Why3 sub-commands and all the realizations and so on, while doing
make install erases the Why3 library for OCaml.