build: fix VPATH build of todo

Without this patch, the make rule in a VPATH build was trying to
invoke ../../docs/../../docs/todo.pl, which didn't exist.

* docs/Makefile.am (todo.html.in): Using $< already implies
$(srcdir) in GNU make VPATH situations.

docs/Makefile.am

@@ -104,7 +104,7 @@ web: $(dot_html) html/index.html devhelp/index.html
 todo.html.in: todo.pl
 	if [ -f  todo.cfg ]; then \
 		echo "Generating $@"; \
-		$(PERL) $(srcdir)/$< > $@ \
+		$(PERL) $< > $@ \
 		|| { rm $@ && exit 1; }; \
 	else \
 		echo "Stubbing $@"; \