conky fails to cross build from source, because it does not find
toluapp. What's happening here is actually a little more involved.
toluapp is actually vendored. It is packaged spearately as
libtolua++5.1-dev albeit for a different (deprecated) lua version. If
conky were using the packaged tolua++ instead of vendoring it, this bug
would fully go away (while leaving #1010294, but that's not your issue).
Unfortunately, tolua++ would have to be upgraded from 5.1 to 5.3 first.
I'm not sure whether that's going to happen. In any case, please do one
of the following:
 * Use a separately packaged tolua++
 * Register your vendored tolua++ with the security tracker at
https://salsa.debian.org/security-tracker-team/security-tracker/-/blob/master/data/embedded-code-copies

Now, I'm going to assume that you register your copy. If you want to
remove the copy, please close this bug with the removal and stop reading
here.

Now conky uses cmake as a build system and cmake is notoriously bad at
distinguishing "build architecture" from "host architecture". Basically,
if you want to build a build tool like tolua++, you need to run cmake
twice somehow (e.g. via an ExternalProject). That doesn't happen here
and it is non-trivial to do. As a consequence, cmake does not consider
the built toluapp relevant and searches for one in $PATH instead where
none is found. Therefore, I'm proposing to extend $PATH with a directory
that contains a separately built toluapp. In essence, that's what the
attached patch does. Please consider applying it. Note that once you do,
conky will continue to fail cross builds until we also fix #1006570, but
again, that's not your issue.

Helmut