V4.1 actually drops support for CPU not having SSE4 support

[antoine-morvan]

Please complete the following tasks.

• Name of seedbox provider or self hosted service
• rTorrent Commit and OS version provided

Tell us about your environment

Tried releases 6.0, 4.1, 4.0 (commit hash associated to each release); under docker image from crazy-max

Actually running on debian 12 > latest docker from docker.io > several crazy-max image versions (alpine 3.19)

Tell us if rTorrent seedbox is self hosted or provided

self hosted

Describe the bug

This commit 8448b61 actually drops support for CPUs that do not have SSE4 instructions. I am not sure this is a desired side effect, but this is not explicitly documented in the README (and I believe such hardware requirement should be explicitly mentionned ^^).

I encountered the issue while updating my instance of rtorrent-rutorrent docker image from crazy-max; that's running on an old dedicated server wiuth an Intel Atom N2800 CPU that does not have SSE4 support. See this issue for more details : crazy-max/docker-rtorrent-rutorrent#396

Not sure about the stance to take on this.

Steps to reproduce

1. Build rtorrent v4.1+ on a CPU that does not have SSE4 support (or run pre-built binaries, such as the one shipped with crazy-max rtorrent-rutorrent image)
2. add a torrent
3. observe error

Expected behavior

I expect rtorrent to support old machines.

Additional context

No response