make distcheck runs a build in dist directory subtirs and then runs intltool -m
which in turn complains about translations in a built file:
The following files contain translations and are currently not in use. Please
consider adding these to the POTFILES.in file, located in the po/ directory.
sub/data/org.freedesktop.ModemManager.policy.in
The issue is reported to intltool upstream already:
https://bugs.launchpad.net/intltool/+bug/1117944