org: avoid displaying the plantuml log during builds

* doc/org/.dir-locals.el.in, doc/org/init.el.in: Make sure plantuml is
started via "java -Djava.awt.headless".

doc/org/.dir-locals.el.in

@@ -35,6 +35,13 @@
 	      (org-babel-C++-compiler . "./g++wrap")
 	      (shell-file-name . "@SHELL@")
               (org-export-html-postamble . nil)
+              (org-babel-default-header-args:plantuml
+               . ((:results . "file")
+                  (:exports . "results")
+                  ; Prevent the plantuml logo to annoying appear on
+                  ; top of whatever you are doing when the project
+                  ; compiles in the background.
+                  (:java . "-Djava.awt.headless")))
 	      (org-publish-project-alist
 	       . (("spot-html"
 		   :base-directory "."

doc/org/init.el.in

@@ -31,6 +31,14 @@
 (setq org-babel-C++-compiler "./g++wrap")
 (setq shell-file-name "@SHELL@")
 
+(setq org-babel-default-header-args:plantuml
+      '((:results . "file")
+        (:exports . "results")
+        ;; prevent the plantuml logo to annoying appear on top of
+        ;; whatever you are doing when the project compiles in the
+        ;; background.
+        (:java . "-Djava.awt.headless")))
+
 (setenv "PATH"
 	(concat "@abs_top_builddir@/bin" path-separator (getenv "PATH")))
 (setenv "PYTHONPATH"