According to the roundcube site https://roundcube.net/about/#features, Apache, Lighttpd, Nginx, Cherokee or Hiawatha are the supported web servers. Gentoo has packages for all of them, yet the ebuild for roundcube forces the use of apache. Please add appropriate local USE variables. Thank you. Reproducible: Always
mail-client/roundcube depends on virtual/httpd-php, which depends on dev-lang/php to have one of 3 flags enabled (in order): fpm, apache2, or cgi. mail-client/roundcube expresses nothing to prefer one server over the other. Only that it needs a working PHP implementation.