Вот эта сборка с LibreSSL проблемная. Без нее (если убрать DEFAULT_VERSIONS+=ssl=libressl из /etc/make.conf или просто поставить пакет) нет ворнингов.

[rihad@master /usr/local/etc/nginx]$ nginx -V
nginx version: nginx/1.16.0
built with LibreSSL 2.9.1
TLS SNI support enabled
configure arguments: --prefix=/usr/local/etc/nginx --with-cc-opt='-I /usr/local/include' --with-ld-opt='-L /usr/local/lib' --conf-path=/usr/local/etc/nginx/nginx.conf --sbin-path=/usr/local/sbin/nginx --pid-path=/var/run/nginx.pid --error-log-path=/var/log/nginx/error.log --user=www --group=www --with-debug --modules-path=/usr/local/libexec/nginx --with-file-aio --http-client-body-temp-path=/var/tmp/nginx/client_body_temp --http-fastcgi-temp-path=/var/tmp/nginx/fastcgi_temp --http-proxy-temp-path=/var/tmp/nginx/proxy_temp --http-scgi-temp-path=/var/tmp/nginx/scgi_temp --http-uwsgi-temp-path=/var/tmp/nginx/uwsgi_temp --http-log-path=/var/log/nginx/access.log --with-http_v2_module --with-http_addition_module --with-http_auth_request_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_realip_module --with-pcre --with-http_secure_link_module --with-http_slice_module --with-http_ssl_module --with-cc-opt='-DNGX_HAVE_INET6=0 -I /usr/local/include' --without-mail_imap_module --without-mail_pop3_module --without-mail_smtp_module --with-stream_ssl_preread_module --with-threads --add-dynamic-module=/usr/ports/www/nginx/work/ngx_brotli-8104036