Splitting this one, to get it started. There's a challenge that our copy of the file is based on older version than any related site (there has been multiple homes for it over the years) has history for. Let's make this ticket about simply establishing a baseline of the file -> something that we say to match known upstream version, with our own modifications. New ticket ( #47652 ) is about then applying/porting upstream updates on top of that.
Patch to establish initial baseline by documenting upstream, and including upstream header as closely matching our version as possible (from the time the file moved from now defunct home to a new location - our copy is some version from that old home)
No functional changes as this touches comments only.
Our mysql.m4 is based on ancient upstream version (or not-so-good upstream between us and autoconf archive - refresh directly from the archive)