automake 1.8 manual indexes versus emacs info

[Kevin Ryde]

Emacs 21 Info-mode doesn't enjoy having the automake indexes in a
sub-node, as is the case in the automake 1.8 manual.  Pressing for
instance "i install" gives an error "No index".

I believe it needs to see menu entries like "Foo Index" or "Bar Index"
in the top-level node.

It's ok for them to be in a sub-node, just so long as there's menu
entries in the top-level, eg. in a "Detailed Node Listing" like the
autoconf manual does.

Personally for just two indexes I wouldn't worry about a sub-node,
just have them in the top-level.  And as @unnumbered rather than
@appendix, since I don't think it's usual to call an index an
appendix.