Hello!

Because package-vc--build-documentation exports the texinfo manual to a
temp file inside /tmp/ , any @include statements with relative paths
break the makeinfo call.

I noticed this issue when attempting to use package-vc to install
org-transclusion, whose manual contains the line

#+texinfo: @include fdl.texi

See: https://raw.githubusercontent.com/nobiot/org-transclusion/main/docs/org-transclusion-manual.org

The attached patch solves this problem by passing the -I flag to
makeinfo. From makeinfo --help:

-I DIR                        append DIR to the @include search path.

Best,

Joseph