autotools: Remove unused code

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
autotools: Use (and provide) convenient m4 macros for plugins

Provided macros support:
- Plugins summary
- Always enabled plugins (for the summary)
- Default enabled/disabled plugins

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>