Build: pass down LIBDIR definition from RPM SPEC to Makefile
    
    Previously the value was sometimes lost. This was causing problems in
    later stages of refactoring.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>