Forgotten doxygen files

doc/img/graphviz/Makefile

+dotfiles := $(wildcard *.dot)
+images := $(patsubst %.dot,%.png,$(wildcard *.dot))
+
+all: $(images)
+	
+%.png: %.dot
+	dot -Tpng $< > $@
+
+.PHONY: clean distclean
+
+clean:
+	-rm $(images)
+
+distclean: clean