Makefile.sphinx: don't build apidoc upon "make apidoc" if SKIP_APIDOC

This is needed because the multiplexer makefile that build apidoc in all
modules explicitly call "make apidoc". Also, it is reasonable to do nothing
anyhow in that case if SKIP_APIDOC=yes is explicitly set.

Makefile.sphinx

@@ -33,7 +33,7 @@ sphinx/clean:
 sphinx/%: $(apidoc_dep)
 	$(SPHINXBUILD) -M $* $(SPHINX_SRCDIR) $(SPHINX_BUILDDIR) $(SPHINXOPTS)
 
-apidoc: apidoc-stamp
+apidoc: $(apidoc_dep)
 apidoc-stamp:
 	$(SPHINXAPIDOC) $(APIDOC_OPTS) -o $(APIDOC_DIR) .. $(APIDOC_EXCLUDES)
 	touch $@