sysutils/xentools413: build failure @pkgsrc-2020Q4 (OCaml has been configured with -force-safe-string: -unsafe-string is not available)

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello together and all belatedly a healthy new year!

I just wanted to ask if this problem is unique to me. When building 
xentools413 from pkgsrc-2020Q4 on NetBSD 9.1 (release) I get an error 
message:

```
gmake[7]: Entering directory 
'/usr/pkgsrc/sysutils/xentools413/work/xen-4.13.2/tools/ocaml/libs/mmap'
ocamldep xenmmap.ml *.mli >.ocamldep.make.new && mv -f 
.ocamldep.make.new .ocamldep.make
ocamlc -unsafe-string -g  -w F -warn-error F -c -o xenmmap.cmi xenmmap.mli
/usr/pkg/bin/ocamlc: OCaml has been configured with -force-safe-string: 
-unsafe-string is not available.
Usage: ocamlc <options> <files>
```

The build will abort after this writeout.

I'll take a closer look tonight. But if there is already a fix for this 
and I just missed it ;-)

Kind regards
Matthias