wireshark: Fix distcheck
    
    Our distcheck is broken. Well, it works but only by pure chance.
    When wireshark plugin is enabled, we try to query which path
    should the plugin be installed into. Firstly, we try to ask
    pkg-config as some releases of wireshark already sets
    corresponding variable in their pkg-config files. However, if we
    obtained no value from there we try to construct the path on our
    own. Based on our observations it usually is:
    $libdir/wireshark/plugins/$version/.
    Now, the problem is in the way we are deciding whether we have
    obtained the plugin directory from pkg-config or not. Simply
    said, we are checking wrong variable. The variable we are
    checking has never been set, thus in our test is empty and
    therefore we will always construct the plugin dir path on our
    own, regardless of its presence in the pkg-config file.
    To make things worse, after fixing this problem, VPATH build was
    broken as it now tried to install plugin into correct directory.
    Yes, this is problem, because --prefix was not honoured and
    everything but the plugin was installed into given prefix. I've
    managed to resolve this issue by replacing plugin dir prefix with
    our own. So when doing regular installation (our prefix ==
    wireshark prefix), nothing changes. When doing VPATH build &
    installation plugin is installed into correctly prefixed dir.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>