diff -u /home/gg/bug/info-look-parent/info-look.el.orig /home/gg/bug/info-look-parent/info-look.el
--- /home/gg/bug/info-look-parent/info-look.el.orig	2012-09-09 07:58:12.000000000 +1000
+++ /home/gg/bug/info-look-parent/info-look.el	2013-02-16 10:47:30.000000000 +1100
@@ -298,6 +298,21 @@
 	(when (string-match (caar file-name-alist) file-name)
 	  (setq info-lookup-mode (cdar file-name-alist)))
 	(setq file-name-alist (cdr file-name-alist)))))
+
+  ;; If major-mode has no setups in info-lookup-alist, under any topic, then
+  ;; search up through derived-mode-parent to find a parent mode which does
+  ;; have some setups.  This means that a `define-derived-mode' with no
+  ;; setups of its own will select its parent mode for lookups, if one of
+  ;; its parents has some setups.  Good for example on `makefile-gmake-mode'
+  ;; and similar derivatives of `makefile-mode'.
+  ;; 
+  (let ((mode major-mode)) ;; look for `mode' with some setups
+    (while (and mode (not info-lookup-mode))
+      (dolist (topic-cell info-lookup-alist) ;; usually only two topics here
+        (if (info-lookup->mode-value (car topic-cell) mode)
+            (setq info-lookup-mode mode)))
+      (setq mode (get mode 'derived-mode-parent))))
+
   (or info-lookup-mode (setq info-lookup-mode major-mode)))
 
 (defun info-lookup-change-mode (topic)

Diff finished.  Sat Feb 16 10:48:55 2013