xentools build failure (ocaml related)

[Hauke Fath <hf%spg.tu-darmstadt.de@localhost>]

On an up-to-date netbsd-9 amd64 installation, both xentools-411 and
xentools-413 builds are failing with

[...]
gmake[7]: Entering directory 
'/var/obj/pkgsrc/sysutils/xentools411/work/xen-4.11.4/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>
[...]

Is this a known issue? Should I send-pr?

Cheerio,
Hauke

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