opendnssec assumes that all ldns >=1.7 have Ed25519/Ed448 but this isn't
correct. In ldns >= 1.8 the constants related to Ed25519/Ed448 are _not_
available unless those features are available, so builds trying to use
them will fail (previously this was a runtime failure).
opendnssec's autoconf script sets USE_ED25519/USE_ED448 (hardcoded to 1)
but doesn't actually use them in code. Change the code to use them, and
hardcode to 0 for now. Should check LDNS_BUILD_CONFIG_USE_ED25519 and
LDNS_BUILD_CONFIG_USE_ED448 on 1.8+.
e2df2949d2