how to makeinfo the standalone texinfo source tarballs?

[David Liu]

I'm trying to read the gcc info manual on emacs, without download the
entire source of gcc. In particular, I've downloaded tarballs at
https://gcc.gnu.org/onlinedocs/ under "texinfo sources of all the gcc
8.3.0" manuals.

However, it is completely unclear what to do with these. Naively attempting
to do `texinfo gcc.texi` fails, complaining about missing files like
`gcc-common.texi`, even though they are easily found in a nearby `include`
directory.

The issue seems to be that since the standalone texinfo source tarball
doesn't contain any Makefiles, the set of manuals cannot be built.

Am I missing something obvious here?