Fix build with gnutls versions 3.0 to 3.2 (Bug#32446)

We previously used functions available only in 3.2+ for all 3.x
versions.
* src/gnutls.c [GNUTLS_VERSION_NUMBER < 0x030501]: Replace calls to
gnutls_cipher_get_tag_size with 0.
[GNUTLS_VERSION_NUMBER < 0x030200]: Alias gnutls_cipher_get_iv_size
to gnutls_cipher_get_block_size, gnutls_digest_list to
gnutls_mac_list, and gnutls_digest_get_name to gnutls_mac_get_name.
[WINDOWSNT]: Adjust DLL function definitions and declarations
accordingly.

src/gnutls.c

@@ -38,6 +38,23 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    So, require 3.5.1.  */
 #if GNUTLS_VERSION_NUMBER >= 0x030501
 # define HAVE_GNUTLS_AEAD
+#else
+/* gnutls_cipher_get_tag_size was introduced in 3.2.0, but it's only
+   relevant for AEAD ciphers.  */
+# define gnutls_cipher_get_tag_size(cipher) 0
+#endif
+
+#if GNUTLS_VERSION_NUMBER < 0x030200
+/* gnutls_cipher_get_iv_size was introduced in 3.2.0.  For the ciphers
+   available in previous versions, block size is equivalent.  */
+#define gnutls_cipher_get_iv_size(cipher) gnutls_cipher_get_block_size (cipher)
+#endif
+
+#if GNUTLS_VERSION_NUMBER < 0x030202
+/* gnutls_digest_list and gnutls_digest_get_name were added in 3.2.2.
+   For previous versions, the mac algorithms are equivalent.  */
+# define gnutls_digest_list() ((const gnutls_digest_algorithm_t *) gnutls_mac_list ())
+# define gnutls_digest_get_name(id) gnutls_mac_get_name ((gnutls_mac_algorithm_t) id)
 #endif
 
 /* gnutls_mac_get_nonce_size was added in GnuTLS 3.2.0, but was
@@ -205,13 +222,21 @@ DEF_DLL_FN (const gnutls_mac_algorithm_t *, gnutls_mac_list, (void));
 DEF_DLL_FN (size_t, gnutls_mac_get_nonce_size, (gnutls_mac_algorithm_t));
 #   endif
 DEF_DLL_FN (size_t, gnutls_mac_get_key_size, (gnutls_mac_algorithm_t));
+#   ifndef gnutls_digest_list
 DEF_DLL_FN (const gnutls_digest_algorithm_t *, gnutls_digest_list, (void));
+#   endif
+#   ifndef gnutls_digest_get_name
 DEF_DLL_FN (const char *, gnutls_digest_get_name, (gnutls_digest_algorithm_t));
+#   endif
 DEF_DLL_FN (gnutls_cipher_algorithm_t *, gnutls_cipher_list, (void));
+#   ifndef gnutls_cipher_get_iv_size
 DEF_DLL_FN (int, gnutls_cipher_get_iv_size, (gnutls_cipher_algorithm_t));
+#   endif
 DEF_DLL_FN (size_t, gnutls_cipher_get_key_size, (gnutls_cipher_algorithm_t));
 DEF_DLL_FN (int, gnutls_cipher_get_block_size, (gnutls_cipher_algorithm_t));
+#   ifndef gnutls_cipher_get_tag_size
 DEF_DLL_FN (int, gnutls_cipher_get_tag_size, (gnutls_cipher_algorithm_t));
+#   endif
 DEF_DLL_FN (int, gnutls_cipher_init,
 	    (gnutls_cipher_hd_t *, gnutls_cipher_algorithm_t,
 	     const gnutls_datum_t *, const gnutls_datum_t *));
@@ -339,13 +364,21 @@ init_gnutls_functions (void)
   LOAD_DLL_FN (library, gnutls_mac_get_nonce_size);
 #   endif
   LOAD_DLL_FN (library, gnutls_mac_get_key_size);
+#   ifndef gnutls_digest_list
   LOAD_DLL_FN (library, gnutls_digest_list);
+#   endif
+#   ifndef gnutls_digest_get_name
   LOAD_DLL_FN (library, gnutls_digest_get_name);
+#   endif
   LOAD_DLL_FN (library, gnutls_cipher_list);
+#   ifndef gnutls_cipher_get_iv_size
   LOAD_DLL_FN (library, gnutls_cipher_get_iv_size);
+#   endif
   LOAD_DLL_FN (library, gnutls_cipher_get_key_size);
   LOAD_DLL_FN (library, gnutls_cipher_get_block_size);
+#   ifndef gnutls_cipher_get_tag_size
   LOAD_DLL_FN (library, gnutls_cipher_get_tag_size);
+#   endif
   LOAD_DLL_FN (library, gnutls_cipher_init);
   LOAD_DLL_FN (library, gnutls_cipher_set_iv);
   LOAD_DLL_FN (library, gnutls_cipher_encrypt2);
@@ -455,13 +488,21 @@ init_gnutls_functions (void)
 #    define gnutls_mac_get_nonce_size fn_gnutls_mac_get_nonce_size
 #   endif
 #  define gnutls_mac_get_key_size fn_gnutls_mac_get_key_size
-#  define gnutls_digest_list fn_gnutls_digest_list
+#  ifndef gnutls_digest_list
-#  define gnutls_digest_get_name fn_gnutls_digest_get_name
+#   define gnutls_digest_list fn_gnutls_digest_list
+#  endif
+#  ifndef gnutls_digest_get_name
+#   define gnutls_digest_get_name fn_gnutls_digest_get_name
+#  endif
 #  define gnutls_cipher_list fn_gnutls_cipher_list
-#  define gnutls_cipher_get_iv_size fn_gnutls_cipher_get_iv_size
+#  ifndef gnutls_cipher_get_iv_size
+#   define gnutls_cipher_get_iv_size fn_gnutls_cipher_get_iv_size
+#  endif
 #  define gnutls_cipher_get_key_size fn_gnutls_cipher_get_key_size
 #  define gnutls_cipher_get_block_size fn_gnutls_cipher_get_block_size
-#  define gnutls_cipher_get_tag_size fn_gnutls_cipher_get_tag_size
+#  ifndef gnutls_cipher_get_tag_size
+#   define gnutls_cipher_get_tag_size fn_gnutls_cipher_get_tag_size
+#  endif
 #  define gnutls_cipher_init fn_gnutls_cipher_init
 #  define gnutls_cipher_set_iv fn_gnutls_cipher_set_iv
 #  define gnutls_cipher_encrypt2 fn_gnutls_cipher_encrypt2