[Preview] Add (barely) FreeBSD build support and update documentation, and potentially improve portability for other UNIX-like platforms

[Renkoto]

Mirror of #99 but for the preview branch.

A noticeable difference is that I needed to add -fPIC to avoid compilation errors with the debug build, I think that this error may also affect any other platform that try to build ssg without LTO.

[Renkoto]

Regarding the audio latency issue, I think that the cause may be related to a known issue with periods and miniaudio in other projects, possibly it is similar in this case, unlike Linux which abandoned OSS a long time ago, most *NIX-like systems use OSS as the standard audio server/interface.

But this will only be true if ssg is using miniaudio as the audio backend by default.