Docs requires doxygen. (#203)

Signed-off-by: Tao He <sighingnow@gmail.com>

docs/Makefile

@@ -28,7 +28,7 @@ clean:
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
-%: Makefile
+%: Makefile doxygen
 	@echo "Tag is:" $(TAG)
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)/$(TAG)" $(SPHINXOPTS) $(O)