Thank you for the quick response, that was very helpful.
For now I added
```
touch doc/tl/tl.tex

touch doc/tl/tl.pdf
```
in my build script and it fixes it, I copied the src to a git repo and then cloned it in the build environment, maybe the timestamps got changed there. I think I will move to downloading and extracting the tarballs from the official link and hopefully I don't need the `touch` command.
Best,
Maxime

On Sat, May 29, 2021 at 11:09 AM Alexandre Duret-Lutz <adl@lrde.epita.fr> wrote:

Maxime Bouton <maxim.bouton@gmail.com> writes:
> I built it on ubuntu (gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0) and
> it did not complain about latexmk, this only happened when changing
> to a new platform (the end goal is to cross compile it).

tl.pdf is included in the tarball, so latexmk should not be needed when
building from the tarball, unless the timestamp of the files were
altered somehow, in which case make might think that tl.pdf is older
than tl.tex. Maybe you manually copied to files to a new location?
(Use "cp -p" to preserve timestamps when copying, or simply extract from
the tarball.)