[docs] Enable creation of dot graphs in the doxygen job.

Author: hageboeck

documentation/doxygen/Makefile

@@ -29,6 +29,7 @@ export DOXYGEN_PYZDOC_PATH := $(DOXYGEN_OUTPUT_DIRECTORY)/pyzdoc
 export DOXYGEN_STRIP_FROM_PATH := $(DOXYGEN_STRIP_PATH) $(DOXYGEN_OUTPUT_DIRECTORY)
 export DOXYGEN_NUM_THREADS := 1
 export DOXYGEN_INPUT_FILTER := ./filter
+export DOXYGEN_HAVE_DOT := YES
 
 PYZ_DIR = $(DOXYGEN_SOURCE_DIRECTORY)/bindings/pyroot/pythonizations/python/ROOT/_pythonization