Instead of generating the list of message ids anew during every build,
the file is generated manually and committed into the
repository. Also, the list of defines is stored in id128-defines.h,
also kept in the repository. Both files should only grow.
This should make build easier. But it also fixes a problem with
systemd, which occasionally drops message definitions. We will keep
them forever, so it should be safe to rely on the presence of message
definitions which systemd does not use anymore.
Fixes #23.