Example apps: Don't use runtime IO config API if CBs are hardcoded

Multiple example applications still use mbedtls_ssl_set_bio() even
if the I/O callbacks are hardcoded. This commit fixes this.
diff --git a/programs/ssl/ssl_fork_server.c b/programs/ssl/ssl_fork_server.c
index c716ca9..098761c 100644
--- a/programs/ssl/ssl_fork_server.c
+++ b/programs/ssl/ssl_fork_server.c
@@ -300,7 +300,14 @@
             goto exit;
         }
 
-        mbedtls_ssl_set_bio( &ssl, &client_fd, mbedtls_net_send, mbedtls_net_recv, NULL );
+#if !defined(MBEDTLS_SSL_CONF_RECV) &&          \
+    !defined(MBEDTLS_SSL_CONF_SEND) &&          \
+    !defined(MBEDTLS_SSL_CONF_RECV_TIMEOUT)
+        mbedtls_ssl_set_bio( &ssl, &client_fd,
+                             mbedtls_net_send, mbedtls_net_recv, NULL );
+#else
+        mbedtls_ssl_set_bio_ctx( &ssl, &client_fd );
+#endif
 
         mbedtls_printf( "pid %d: SSL setup ok\n", pid );