fix parallel make
https://bugs.gentoo.org/show_bug.cgi?id=422683

Index: odns-0.3/OCamlMakefile
===================================================================
--- odns-0.3.orig/OCamlMakefile
+++ odns-0.3/OCamlMakefile
@@ -1152,7 +1152,7 @@ $(BCDIDIR)/%.di $(NCDIDIR)/%.di:	%.mli
 $(DOC_DIR)/$(RESULT)/html:
 	mkdir -p $@
 
-$(DOC_DIR)/$(RESULT)/html/index.html: $(DOC_DIR)/$(RESULT)/html $(DOC_FILES)
+$(DOC_DIR)/$(RESULT)/html/index.html: $(DOC_DIR)/$(RESULT)/html $(DOC_FILES) byte-code-library
 	rm -rf $</*
 	$(QUIET)pp=`sed -n -e '/^#/d' -e 's/(\*pp \([^*]*\) \*)/\1/p;q' $(FIRST_DOC_FILE)`; \
 	if [ -z "$$pp" ]; then \