Track Makefile.am legacy directive cleanup

## Context
PR #498 removes legacy/redundant automake directives in `Makefile.am`.

## Scope
- remove `ACLOCAL_AMFLAGS = -I m4`
- remove redundant `EXTRA_DIST = spine.1`

## Acceptance Criteria
- `./bootstrap` succeeds
- configure/build/check pipeline remains green

## Linked PR
- #498