Let's say you don't have a DOC_MAIN_SGML_FILE in your source tree and wants to use the default one.
Building your project using "make" works fine but using "make -j3" doesn't. You'll get this kind of error:

*** No rule to make target `wocky-docs.sgml', needed by `html-build.stamp'

This is because if this file doesn't exist, it's generated during the gtkdoc-mkdb call in the XML section. But gtk-doc.make doesn't reflect this dependency and so the build fails when running in parallel.

I fixed this problem by changing the rules in the XML section like that:

#### xml ####

$(DOC_MAIN_SGML_FILE): tmpl.stamp $(HFILE_GLOB) $(CFILE_GLOB) $(DOC_MODULE)-sections.txt $(srcdir)/tmpl/*.sgml $(expand_content_files)
	@echo 'gtk-doc: Building XML'
	@-chmod -R u+w $(srcdir)
	cd $(srcdir) && \
	gtkdoc-mkdb --module=$(DOC_MODULE) --source-dir=$(DOC_SOURCE_DIR) --output-format=xml --expand-content-files="$(expand_content_files)" --main-sgml-file=$(DOC_MAIN_SGML_FILE) $(MKDB_OPTIONS)

sgml-build.stamp: $(DOC_MAIN_SGML_FILE)
	touch sgml-build.stamp