Warning about deprecated distutils-r1.eclass legacy mode

```
 * distutils-r1.eclass legacy mode is deprecated and will be removed.
 * Please migrate your ebuilds to use DISTUTILS_USE_PEP517 (common values
 * are 'setuptools' for packages using setuptools/distutils,
 * and 'no' for packages using non-PEP517 build systems).
```