Use OpenSSL Ed25519 implementation only when possible
Only switch to OpenSSL Ed25519 implementation if all the requirements are satisfied. This means:
- OpenSSL has the single shot
EVP_DigestSign()
function - OpenSSL has the single shot
EVP_DigestVerify()
function - OpenSSL has the
EVP_PKEY_ED25519
type inopenssl/evp.h
This fixes T197
Edited by Anderson Sasaki