doc/doxygen_dev/Makefile.am: add rule to create html_dev directory

doc/doxygen_dev/Makefile.am

@@ -109,6 +109,9 @@ dox_inputs = $(DOX_CONFIG) 				\
 	chapters/version.html				\
 	$(top_srcdir)/src/core/workers.h
 
+$(DOX_HTML_DIR): $(DOX_TAG) refman.tex
+       @$(MKDIR_P) $(DOX_HTML_DIR)
+
 $(DOX_TAG): $(dox_inputs)
 	@rm -fr $(DOX_HTML_DIR) $(DOX_LATEX_DIR)
 	@$(DOXYGEN) $(DOX_CONFIG)