C-h S in the various makefile-mode derivatives such as
makefile-gmake-mode for GNUmakefile prompts for

    Use symbol help mode:

where I hoped it would already know what to do.

Perhaps something like the following, also confining the automake manual
things to the automake mode.  It and the make manual overlap in a few
places, mainly variables like LFLAGS etc.

2011-05-11  Kevin Ryde  <user42@zip.com.au>

	* info-look.el (makefile-automake-mode): New setups, looking in
	automake manual, then makefile-mode.
	(makefile-mode): Remove automake manual, have it just in
	makefile-automake-mode.  Remove non-existent automake-mode,
	believe a hypothetical automake-mode would go to makefile-mode,
	not the other way around.
	(makefile-bsdmake-mode, makefile-gmake-mode, makefile-imake-mode)
	(makefile-makepp-mode): New setups, just going to makefile-mode.
	In particular makefile-gmake-mode lets lookups work from a
	GNUmakefile.