Modify is_lasso_ecp_enabled() to test for library symbol

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>