regression in dune lang 1.0 semantics of cxx_ fields due to changes in `select` semantics

[shonfeder]

Expected Behavior

Backward compatibility is maintained for packages using (lang dune 1.0).

Actual Behavior

Building mccs.1.1+19 fails with

error: Cannot build '/nix/store/6rkvhwwnyhhblr960wmy7mc7x65xgmf0-ocaml5.4.0-mccs-1.1+19.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/z3lxd3blzigpv22sl3zhh01a5vcb6rmh-ocaml5.4.0-mccs-1.1+19
       Last 18 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/892sm7xb7rgif2svxficpr9lpszgf9db-source
       > source root is source
       > Running phase: patchPhase
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > no configure script, doing nothing
       > Running phase: buildPhase
       > File "src/dune", lines 20-26, characters 2-171:
       > 20 |   (select
       > 21 |    config.h
       > 22 |    from
       > 23 |    (mccs.glpk.internal -> config_internal.h)
       > 24 |    (mccs.glpk.shared -> config_external.h)
       > 25 |    (mccs.glpk.static -> config_external.h)
       > 26 |    (-> _))))
       > Error: No solution found for this select form.
       >
       For full logs, run:
         nix log /nix/store/6rkvhwwnyhhblr960wmy7mc7x65xgmf0-ocaml5.4.0-mccs-1.1+19.drv

Reproduction

opam pin dune.dev --dev-repo
opam install mccs.1.1+19

Specifications

• Version of dune (output of dune --version): trunk

Additional information

Running the revdeps bisect action identified #13175 as the PR that introduced the regression in 75b52fa

Identified via the revdeps testing for the dune 3.22 release candidate: ocaml/opam-repository#29535 (comment)