Update function and variable names that are obsolete since Emacs 26.
Don't try to configure flymake without a jing jar file.

--- rnc-mode-1.0.6/rnc-mode.el
+++ rnc-mode-1.0.6/rnc-mode.el
@@ -113,8 +113,8 @@
 
 (defun rnc-flymake-init ()
   "Flymake init function for running Jing on the schema."
-  (let* ((temp-file (flymake-init-create-temp-buffer-copy
-                     'flymake-create-temp-inplace))
+  (let* ((temp-file (flymake-proc-init-create-temp-buffer-copy
+                     'flymake-proc-create-temp-inplace))
          (local-file (file-relative-name
                       temp-file
                       (file-name-directory buffer-file-name))))
@@ -123,11 +123,11 @@
 (defun rnc-configure-flymake ()
   (if (and (stringp rnc-jing-jar-file)
            (file-exists-p rnc-jing-jar-file))
-      (add-to-list 'flymake-allowed-file-name-masks
+      (add-to-list 'flymake-proc-allowed-file-name-masks
                    '(".+\\.rnc$"
                      rnc-flymake-init
-                     flymake-simple-cleanup
-                     flymake-get-real-file-name))
+                     flymake-proc-simple-cleanup
+                     flymake-proc-get-real-file-name))
     (message "RNC flymake not enabled because the Jing jar could not be found")))
 
 ;;; Imenu support
@@ -329,8 +329,9 @@
     (modify-syntax-entry ?_ "w   " rnc-syntax-table)
     (set-syntax-table rnc-syntax-table))
 
-  (rnc-configure-flymake)
-  (when rnc-enable-flymake
-    (flymake-mode))
+  (when rnc-jing-jar-file
+    (rnc-configure-flymake)
+    (when rnc-enable-flymake
+      (flymake-mode)))
   (when rnc-enable-imenu
     (rnc-configure-imenu))