I have severe problems installing some packages with opam. I want to 
install 'js_of_ocaml' with opam, but it cannot install the package 
'conf-pkg-config'. In trying to do this I get the error:

    opam install conf-pkg-config
    The following actions will be performed:
       ∗  install conf-pkg-config 1.0

    =-=- Gathering sources
    =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

    =-=- Processing actions
    -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
    [ERROR] The compilation of conf-pkg-config failed at "pkg-config
    --help".

    #=== ERROR while installing conf-pkg-config.1.0
    ===============================#
    # opam-version 1.2.2
    # os           linux
    # command      pkg-config --help
    # path /home/helmut/.opam/4.02.3/build/conf-pkg-config.1.0
    # compiler     4.02.3
    # exit-code    127
    # env-file
    /home/helmut/.opam/4.02.3/build/conf-pkg-config.1.0/conf-pkg-config-11348-42bb52.env
    # stdout-file
    /home/helmut/.opam/4.02.3/build/conf-pkg-config.1.0/conf-pkg-config-11348-42bb52.out
    # stderr-file
    /home/helmut/.opam/4.02.3/build/conf-pkg-config.1.0/conf-pkg-config-11348-42bb52.err



    =-=- Error report
    -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
    The following actions failed
       ∗  install conf-pkg-config 1.0
    No changes have been performed

    =-=- conf-pkg-config.1.0 troobleshooting
    =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
    => This package relies on external (system) dependencies that may be
    missing. `opam depext conf-pkg-config.1.0' may help you find the
        correct installation for your system.


The recommended check does not give any information.

I have installed the version 4.02.3 successfully via opam and switched 
to that version.

Any ideas on how to analyze the problem?

Regards
Helmut