devel/agar: Update 1.7.0 => 1.7.1, improve port
Changelog:
https://github.com/JulNadeauCA/libagar/blob/trunk/CHANGELOG.md#171---2025-11-15
Improve port:
- Replace PORTVERSION with DISTVERSION.
- Remove GNU_CONFIGURE_MANPREFIX.
- Convert SDL*_PREVENTS to OPTIONS_RADIO_SDL.
- Fix warnings from portclippy.
PR: 291038
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>