Well,
I can't manage to do it.
Something is curious: the directories own to root
ls -ali /var/lib/letsencrypt
3801125 drwxr-x--- 4 root root 4096 août 15 18:36 .
3670018 drwxr-xr-x 82 root root 4096 juil. 28 13:36 ..
3801925 drwxr-x--- 3 root root 4096 mai 5 2016 backups
3802591 drwxr-xr-x 2 root root 4096 août 15 21:11 http_challenges
so I did: (I presume that www-data, the httpd user must get R/W rights
chgrp -R www-data /var/lib/letsencrypt
chmod g+w -R /var/lib/letsencrypt
then I add what is created by certbot into the VHost conf:
Alias /.well-known /www/.well-known
<Location /.well-known/acme-challenge>
Require all granted
</Location>
<Directory /var/lib/letsencrypt/http_challenges>
Require all granted
</Directory>
but I was unable to get the test file from this new location
I also tried with following command without success
# ./letsencrypt-auto --test-cert --webroot-path /www/.well-known/acme-challenge --apache certonly -d phlinux.ovh