- New upstream major release with tons of new features and LTS (see: https://www.haproxy.com/blog/announcing-haproxy-2-4/)
- Update haproxy download URL and hash
- Activate promtheus exporter support the new way (using USE_PROMEX=1)
- Cleaned up haproxy-specific CFLAGS
- Changed the halog build to make use of the new Makefile target (admin/halog/halog)

Signed-off-by: Christian Lachner <gladiac@gmail.com>