For example:
```
meson builddir \
--native-file vs2019-paths.txt \
--native-file vs2019-win-x64.txt \
--cross-file vs2019-paths.txt \
--cross-file vs2019-win-arm64.txt
```
This was causing the error:
> ERROR: Multiple producers for Ninja target "/path/to/vs2019-paths.txt". Please rename your targets.
Fix it by using a set() when generating the list of regen files, and
add a test for it too.