gnat_rm.texi and makeinfo 5.1

[Arnaud Charlet]

> cc'ing Eric B since I remember him mentioning that they autogenerated
> texinfo from Sphinx and I am wondering if this is manual or autogenerated.

No, that was me, mentioning that we were planning to do that in the future,
but not yet.

> Other than fixing the manual or downgrading makeinfo, any suggestions
> or comments?

makeinfo 4.8 does work indeed. I'm sure there's a simple way to modify
the syntax here to be suitable for more recent versions of makeinfo.

Arno