Arch Linux

Description:

I maintain the cppo package on AUR, and a user reported being unable to build the package using the latest version of dune (3.0.3-1). cppo depends on ocamlbuild, but dune was unable to find the system-installed version (a locally installed version via opam was being picked up).

Comparing the output of `ocamlfind list`and `dune installed-libraries` gives:
```
bigarray (version: [distributed with Ocaml])
bytes (version: [distributed with OCaml 4.02 or above])
compiler-libs (version: [distributed with Ocaml])
compiler-libs.bytecomp (version: [distributed with Ocaml])
compiler-libs.common (version: [distributed with Ocaml])
compiler-libs.optcomp (version: [distributed with Ocaml])
compiler-libs.toplevel (version: [distributed with Ocaml])
csexp (version: n/a)
dune (version: n/a)
dune-action-plugin (version: n/a)
dune-build-info (version: n/a)
dune-configurator (version: n/a)
dune-glob (version: n/a)
dune-private-libs (version: n/a)
dune-private-libs.dune-section (version: n/a)
dune-private-libs.dune_re (version: n/a)
dune-private-libs.meta_parser (version: n/a)
dune-private-libs.ocaml-config (version: n/a)
dune-site (version: n/a)
dune-site.plugins (version: n/a)
dune.configurator (version: n/a)
dyn (version: n/a)
dyn.pp (version: n/a)
dynlink (version: [distributed with Ocaml])
fiber (version: n/a)
findlib (version: 1.9.1)
findlib.dynload (version: 1.9.1)
findlib.internal (version: 1.9.1)
findlib.top (version: 1.9.1)
ocamlbuild (version: 0.14.0)
ocamldoc (version: [distributed with Ocaml])
ordering (version: n/a)
pp (version: n/a)
result (version: n/a)
stdlib (version: [distributed with Ocaml])
stdune (version: n/a)
stdune.csexp (version: n/a)
stdune.filesystem_stubs (version: n/a)
str (version: [distributed with Ocaml])
threads (version: [distributed with Ocaml])
threads.none (version: [internal])
threads.posix (version: [internal])
unix (version: [distributed with Ocaml])
xdg (version: n/a)
```

and for dune:
```
bigarray (version: [distributed with OCaml])
bytes (version: [distributed with OCaml])
compiler-libs (version: [distributed with OCaml])
compiler-libs.bytecomp (version: [distributed with OCaml])
compiler-libs.common (version: [distributed with OCaml])
compiler-libs.optcomp (version: [distributed with OCaml])
compiler-libs.toplevel (version: [distributed with OCaml])
dynlink (version: [distributed with OCaml])
ocamldoc (version: [distributed with OCaml])
stdlib (version: [distributed with OCaml])
str (version: [distributed with OCaml])
threads (version: [distributed with OCaml])
threads.posix (version: [distributed with OCaml])
unix (version: [distributed with OCaml])
```

In the latter output clearly dune just doesn't find ocamlbuild. I submitted an issue to cppo upstream (https://github.com/ocaml-community/cppo/issues/82) and it was pointed out to me that the ArchLinux dune package does not explicitly set the system libdir (as is done by Debian). I am unsure why this matters, I understood dune should be able to auto-detect this, and if we look at the verbose output of `dune installed-libraries` we get a list of envs such as `"DUNE_OCAML_STDLIB" : "/usr/lib/ocaml"` which to me at least means it know where to look for stdlib libraries.

At any rate, before submitting anything here I thought I would have a go at compiling dune *with* an explicit libdir at configure time (see attached PKGBUILD; based off of what debian packagers do: https://salsa.debian.org/ocaml-team/ocaml-dune/-/blob/master/debian/rules#L22). Upon building and installing, dune behaves as expected.

The output of `dune installed-libraries` is now:
```
bigarray (version: [distributed with OCaml])
bytes (version: [distributed with OCaml])
compiler-libs (version: [distributed with OCaml])
compiler-libs.bytecomp (version: [distributed with OCaml])
compiler-libs.common (version: [distributed with OCaml])
compiler-libs.optcomp (version: [distributed with OCaml])
compiler-libs.toplevel (version: [distributed with OCaml])
csexp (version: n/a)
dune-action-plugin (version: n/a)
dune-build-info (version: n/a)
dune-configurator (version: n/a)
dune-glob (version: n/a)
dune-private-libs.dune-section (version: n/a)
dune-private-libs.dune_re (version: n/a)
dune-private-libs.meta_parser (version: n/a)
dune-private-libs.ocaml-config (version: n/a)
dune-site (version: n/a)
dune-site.plugins (version: n/a)
dune.configurator (version: n/a)
dyn (version: n/a)
dyn.pp (version: n/a)
dynlink (version: [distributed with OCaml])
fiber (version: n/a)
findlib (version: 1.9.1)
findlib.dynload (version: 1.9.1)
findlib.internal (version: 1.9.1)
findlib.top (version: 1.9.1)
ocamlbuild (version: 0.14.0)
ocamldoc (version: [distributed with OCaml])
ordering (version: n/a)
pp (version: n/a)
result (version: n/a)
stdlib (version: [distributed with OCaml])
stdune (version: n/a)
stdune.csexp (version: n/a)
stdune.filesystem_stubs (version: n/a)
str (version: [distributed with OCaml])
threads (version: [distributed with OCaml])
threads.posix (version: [distributed with OCaml])
unix (version: [distributed with OCaml])
xdg (version: n/a)
```

Steps to reproduce:

```
git clone https://github.com/ocaml-community/cppo.git
cd cppo
dune build --profile release
```