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.

Fixes: T197

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

ConfigureChecks.cmake

@@ -148,7 +148,12 @@ if (OPENSSL_FOUND)
 
     set(CMAKE_REQUIRED_INCLUDES ${OPENSSL_INCLUDE_DIR})
     set(CMAKE_REQUIRED_LIBRARIES ${OPENSSL_CRYPTO_LIBRARY})
-    check_symbol_exists(EVP_PKEY_ED25519 "openssl/evp.h" HAVE_OPENSSL_ED25519)
+    check_symbol_exists(EVP_PKEY_ED25519 "openssl/evp.h" FOUND_OPENSSL_ED25519)
+
+    if (HAVE_OPENSSL_EVP_DIGESTSIGN AND HAVE_OPENSSL_EVP_DIGESTVERIFY AND
+        FOUND_OPENSSL_ED25519)
+        set(HAVE_OPENSSL_ED25519 1)
+    endif()
 
     set(CMAKE_REQUIRED_INCLUDES ${OPENSSL_INCLUDE_DIR})
     set(CMAKE_REQUIRED_LIBRARIES ${OPENSSL_CRYPTO_LIBRARY})