[help-texinfo] Generate an index of texinfo manual

[reader]

I've been pawing thru this group and googling to find a few commands
to generate an index from a manual already in texinfo format.

Lots of hits but not really seeing how its done... Instead I find
lengthy complicated descriptions of index types and etc.

I usually use the info browser thru emacs and I've found some of the
entries in the manual (gawk comes to mind) are not indexed.  Or I
guess that is what I'm seeing.

  C h i m gawk <RET>

Brings up the gawk manual alright but then pressing `i' for an index
search tells me `no index'.  I'm pretty sure in the past gawk had an
index since I recall looking up the printf function a few times.  But
long long ago.  Perhaps this means something is wrong with my
installation of texinfo?

I'm seem to recall having generated indexs in the past and I know
nearly nothing about texinfo so it must not have been too involved.

How can I create an index so that pressing `i' in gawk manual allows
me to do an index search?