Detect crypto library BLAKE2 support

While OpenSSL supports both "b" and "s" variants of the BLAKE2 hash
function, other cryptographic libraries may lack support for one or both
of the variants. This commit modifies `hashlib`'s C code to detect
whether or not the linked libcrypto supports each BLAKE2 variant, and
elides references to each variant's NID accordingly. In cases where the
underlying libcrypto doesn't fully support BLAKE2, CPython's
`./configure` script can be given the following flag to use CPython's
interned BLAKE2 implementation: `--with-builtin-hashlib-hashes=blake2`.

Author: WillChilds-Klein

Modules/_hashopenssl.c

@@ -47,7 +47,9 @@
 #define PY_OPENSSL_HAS_SCRYPT 1
 #define PY_OPENSSL_HAS_SHA3 1
 #define PY_OPENSSL_HAS_SHAKE 1
+#if defined(NID_blake2s256) || defined(NID_blake2b512)
 #define PY_OPENSSL_HAS_BLAKE2 1
+#endif
 
 #if OPENSSL_VERSION_NUMBER >= 0x30000000L
 #define PY_EVP_MD EVP_MD
@@ -102,8 +104,12 @@ typedef struct {
 #define Py_hash_sha3_512 "sha3_512"
 #define Py_hash_shake_128 "shake_128"
 #define Py_hash_shake_256 "shake_256"
+#if defined(NID_blake2s256)
 #define Py_hash_blake2s "blake2s"
+#endif
+#if defined(NID_blake2b512)
 #define Py_hash_blake2b "blake2b"
+#endif
 
 #define PY_HASH_ENTRY(py_name, py_alias, ossl_name, ossl_nid) \
     {py_name, py_alias, ossl_name, ossl_nid, 0, NULL, NULL}
@@ -130,8 +136,12 @@ static const py_hashentry_t py_hashes[] = {
     PY_HASH_ENTRY(Py_hash_shake_128, NULL, SN_shake128, NID_shake128),
     PY_HASH_ENTRY(Py_hash_shake_256, NULL, SN_shake256, NID_shake256),
     /* blake2 digest */
+#if defined(NID_blake2s256)
     PY_HASH_ENTRY(Py_hash_blake2s, "blake2s256", SN_blake2s256, NID_blake2s256),
+#endif
+#if defined(NID_blake2s256)
     PY_HASH_ENTRY(Py_hash_blake2b, "blake2b512", SN_blake2b512, NID_blake2b512),
+#endif
     PY_HASH_ENTRY(NULL, NULL, NULL, 0),
 };