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)

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

I've fixed that using memcmp, which should be fine since compared elements
are of the same types.

Also, the simulation via the _loop.c file was failing in extern types were present
as O/I of the main (simulated) node.

when calling soc.

Do not work all the times (eg, with arrays it generates C code that
does not compile). cf next changes.

i.e., by providing a default macro definition that uses memcpy()
(versus generating a nop function).

The rationale is to prepare the work for a future --2s-stack option.