autogen 5.18.2

Updated Autogen to 5.18.2 and inserts # explanation as to why the update isn't and shouldn't be to the newest version.

See [this PR](https://github.com/Homebrew/homebrew/pull/30406) for full information on the ongoing 5.18.3 'guile' issues.
autogen: add 5.18.2 bottle.

Closes #30495.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
Convert to pathname before calling relative_path_from

Fixes #30494.
update libgit2 to v0.21.0
libgit2: add 0.21.0 bottle.