ipa-kdb: support Samba 4.20 private libraries
    
    Samba 4.20 will change name extension of the private libraries from
    'samba4' to 'private-samba'. Detect private extension through configure
    step and make sure to use the right library name in Makefile.
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>