In the -2c mode, in presence of extern types, I was defining them as int.
The idea was to have something that compiles, and then to ask the user
to redefine its extern types.

Now I define extern types as string (char*).

The rationale of this change is that before, the generated code was only
working for int, which is pretty useless as an extern type.

Have string by default is actually much more useful, for instance to use
string in a Lustre program.

Also, adapt to changes done in lutils, where the string type has ben added.
(sha:8619aa285ca8322ab9, i.e., > lutils  1.49.10)

(It would have avoided issue #8)

close #8

Remove a lot of warnings (considered as errors by dune).

cd rdbg #sha 4a54c7a1181e1200670f3eaf14af1be32823cd04

Indeed the API of Format.formatter_out_functions changed in 4.06.0

To turn around this problem, I've used pp_get_formatter_out_functions
and extend it result

Merge branch 'pascal01'