Bug#1042098: ocaml-gettext FTBFS with ocaml-dune 3.9.1

[Adrian Bunk <bunk@debian.org>]

Source: ocaml-gettext
Version: 0.4.2-1
Severity: serious
Tags: ftbfs trixie sid

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/ocaml-gettext.html

...
   debian/rules override_dh_auto_install
make[1]: Entering directory '/build/1st/ocaml-gettext-0.4.2'
mkdir -p /build/1st/ocaml-gettext-0.4.2/debian/tmp//usr/lib/ocaml
/usr/bin/make install DESTDIR=/build/1st/ocaml-gettext-0.4.2/debian/tmp \
	prefix=/build/1st/ocaml-gettext-0.4.2/debian/tmp/usr sysconfdir=/build/1st/ocaml-gettext-0.4.2/debian/tmp/etc \
	OCAMLFIND_LDCONF=ignore \
	DOCDIR='/build/1st/ocaml-gettext-0.4.2/debian/tmp/usr/share/doc/libgettext-ocaml-dev'
make[2]: Entering directory '/build/1st/ocaml-gettext-0.4.2'
dune build @install
dune install
Error: The mandir installation directory is unknown.
Hint: It can be specified with --prefix or by setting --mandir
make[2]: *** [Makefile:46: install] Error 1