Hard-coded command for generating `version.ml` triggers compiler warnings

[nberth]

The warning bellow appears twice for each version.mlt in a project, when a recent version of OCaml is used (5.2.1 at least).

File "_none_", line 1:
Alert ocaml_deprecated_auto_include: 
OCaml's lib directory layout changed in 5.0. The unix subdirectory has been
automatically added to the search path, but you should add -I +unix to the
command-line to silence this alert (e.g. by adding unix to the list of
libraries in your dune file, or adding use_unix to your _tags file for
ocamlbuild, or using -package unix for ocamlfind).

These warnings are just annoying but do not prevent compilation.

Still, this issue could be fixed before #234 is addressed.