cmake: Only use OpenSSL Ed25519 implementation when possible
It is possible for OpenSSL to have the type EVP_PKEY_ED25519 in
openssl/evp.h, but not the single shot functions EVP_DigestSign() and
EVP_DigestVerify() which are required to generate Ed25519 signatures.
Only switch to use OpenSSL Ed25519 implementation when OpenSSL have all
required types and functions.
Signed-off-by: Anderson Toshiyuki Sasaki <email@example.com>
Reviewed-by: Andreas Schneider <firstname.lastname@example.org>