Modify is_lasso_ecp_enabled() to test for library symbol
    
    The function is_lasso_ecp_enabled() in saml2idp.py was testing for a
    specific lasso version. That has proved problematic because we've had
    too many "unofficial" versions floating around and sometimes the
    version check produces the wrong result.
    
    A better test is to test for the presence of a symbol in the lasso
    library we know will only be there with full ECP support.
    
    Now we test for the presence of ECP_ERROR_MISSING_AUTHN_REQUEST.
    
    Ticket: 167
    Signed-off-by: John Dennis <jdennis@redhat.com>
    Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>