make java executable configurable for ditaa blocks

* lisp/ob-ditaa.el (org-babel-ditaa-java-cmd): Make java executable
  configurable for ditaa blocks.

lisp/ob-ditaa.el

@@ -58,6 +58,11 @@
   :group 'org-babel
   :type 'string)
 
+(defcustom org-babel-ditaa-java-cmd "java"
+  "Java executable to use when evaluating ditaa blocks."
+  :group 'org-babel
+  :type 'string)
+
 (defcustom org-ditaa-eps-jar-path
   (expand-file-name "DitaaEps.jar" (file-name-directory org-ditaa-jar-path))
   "Path to the DitaaEps.jar executable."
@@ -86,7 +91,8 @@ This function is called by `org-babel-execute-src-block'."
 	 (java (cdr (assoc :java params)))
 	 (in-file (org-babel-temp-file "ditaa-"))
 	 (eps (cdr (assoc :eps params)))
-	 (cmd (concat "java " java " " org-ditaa-jar-option " "
+	 (cmd (concat org-babel-ditaa-java-cmd
+		      " " java " " org-ditaa-jar-option " "
 		      (shell-quote-argument
 		       (expand-file-name
 			(if eps org-ditaa-eps-jar-path org-ditaa-jar-path)))