Define TLS1_3_VERSION as invalid value if missed

The patch fixes build with OpenSSL < 1.1.1:
./ctx.go:372:28: could not determine kind of name for C.TLS1_3_VERSION

Author: oleg-jukovec

shim.h

@@ -67,6 +67,9 @@ extern int X_SSL_verify_cb(int ok, X509_STORE_CTX* store);
 
 /* SSL_CTX methods */
 extern int X_SSL_CTX_new_index();
+#ifndef TLS1_3_VERSION
+#define TLS1_3_VERSION 0
+#endif
 extern int X_SSL_CTX_set_min_proto_version(SSL_CTX *ctx, int version);
 extern int X_SSL_CTX_set_max_proto_version(SSL_CTX *ctx, int version);
 extern long X_SSL_CTX_set_options(SSL_CTX* ctx, long options);
@@ -181,4 +184,4 @@ extern int OBJ_create(const char *oid,const char *sn,const char *ln);
 
 /* Extension helper method */
 extern const unsigned char * get_extention(X509 *x, int NID, int *data_len);
-extern int add_custom_ext(X509 *cert, int nid, char *value, int len);
+extern int add_custom_ext(X509 *cert, int nid, char *value, int len);