building mes file is broken

In building kea we can notice an error:

make[5]: *** No rule to make target 'dhcp6_messages.mes', needed by 'dhcp6_messages.cc'.  Stop.
make[5]: *** Waiting for unfinished jobs....

Full log: https://jenkins.isc.org/job/kea-1.7/job/ut-basic/87/execution/node/165/log/

It is also visible while building packages:

Exception occurred:
  File "/home/vagrant/kea-src/kea-1.7.3/doc/sphinx/mes2doc.py", line 39, in read_input_files
    with open(f) as fp:
FileNotFoundError: [Errno 2] No such file or directory: '/home/vagrant/kea-src/kea-1.7.3/src/bin/dhcp6/dhcp6_messages.mes'

Full log: https://jenkins.isc.org/job/kea-1.7/job/pkg/121/execution/node/93/log/