Repro test: no hint about missing depext

test/blackbox-tests/test-cases/pkg/depexts/system-deps.t

@@ -0,0 +1,77 @@
+If we have a 'pure' system dependency (no OCaml compilation unit)
+and no 'libraries' field, we don't fail nor do we print a hint
+
+  $ . ../helpers.sh
+  $ mkrepo
+  $ add_mock_repo_if_needed
+
+  $ cat > dune-project << EOF
+  > (lang dune 3.20)
+  > (package
+  >  (name foo)
+  >  (depends conf-foobar))
+  > EOF
+
+  $ mkdir src
+
+  $ cat > src/dune << EOF
+  > (executable
+  >  (name foo)
+  >  (public_name foo))
+  > EOF
+
+  $ cat > src/foo.ml << EOF
+  > let _exit_code = Sys.command "foobar some_arg" in
+  > ()
+  > EOF
+
+  $ mkpkg conf-foobar << EOF
+  > build: [
+  >   ["foobar" "--version"]
+  > ]
+  > depexts: [
+  >   ["foobar-cram"] 
+  > ]
+  > flags: conf
+  > EOF
+
+Dune is aware of the dependency
+  $ dune show depexts
+  foobar-cram
+
+And we add it to the lock solution
+  $ dune pkg lock
+  Solution for dune.lock
+
+  Dependencies common to all supported platforms:
+  - conf-foobar.0.0.1
+
+Yet we don't try to build it at all
+  $ dune build
+
+And we don't print the hint
+  $ dune exec src/foo.exe
+  sh: line 1: foobar: command not found
+
+If however, the 'libraries' field is present and there is an unknown 'hi'
+lib required, dune will try building depexts to find said 'hi' module,
+even if it is unrelated to our depext.
+
+  $ rm src/dune
+  $ cat > src/dune << EOF
+  > (executable
+  >  (name foo)
+  >  (public_name foo)
+  >  (libraries hi))
+  > EOF
+
+Now we see it!
+  $ dune build
+  File "dune.lock/conf-foobar.0.0.1.pkg", line 4, characters 30-36:
+  4 |  (all_platforms ((action (run foobar --version)))))
+                                    ^^^^^^
+  Error: Program foobar not found in the tree or in PATH
+   (context: default)
+  Hint: You may want to verify the following depexts are installed:
+  - foobar-cram
+  [1]