Simplify obtaining of PKGDIR
    
    So far, in all Makefiles we have a complicated way to construct
    PKGDIR path which is used to install python modules into.
    Firstly, python version is obtained, and then system prefix and
    these are then used to construct the PKGDIR path. Well, we can do
    better. We can use 'site.getsitepackages()[0]' to obtain exactly
    the path we are after.
    
    This also fixes the problem on distributions which have split
    /usr/lib and /usr/lib64 directories, because with the way we are
    constructing the PKGDIR path we assume that the directories are
    the same (or one is a symlink to another). Well, that is not
    always the case.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>