The gnutls_digest_get_id function was added in GnuTLS 3.2.2:

https://gitlab.com/gnutls/gnutls/blob/gnutls_3_2_2/NEWS

That was released in 2013, so I think it's OK to depend on it by now. Please consider applying this patch to update the configure.in check:

https://github.com/openldap/openldap/compare/master...rtandy:its8753-gnutls-...

Currently master fails to compile with older GnuTLS (e.g. on Debian 7/wheezy) after configure succeeded.