My understanding is that bundling / vendoring dependencies should be fine since the resulting output has to be GPLv3 anyways (so the dependencies have to have a GPLv3 forward-compatible license). Of course, this isn't legal advice.
In case you missed it, I do have some basic build tooling in the form of
a project template that generates Ninja build scripts, which I use for rebuilding large projects. Definitely doesn't do any of the fancy dependency lookups that your tool appears to accomplish, though.