/etc/mail/m4/opendkim.m4 is in opendkim 2.11.0~alpha-11build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | INPUT_MAIL_FILTER(`opendkim',
`S=local:/var/run/opendkim/opendkim.sock, F=, T=S:4m;R:4m;E:10m')dnl
|