Make Godot compile on FreeBSD

[MBCX]

Fixes #67838, well, at least it does build on my dedicated FreeBSD 12.4 VM.

Some issues however:

1. There are minor thirdparty code modifications, hopefully its acceptable. Committed the changed and also documented them in a diff file.
2. Not a fan of having to import sys directly to specifically detect FreeBSD. Any other solution would be appreciated. Seems like the simplest solution for now.

I'll tackle OpenBSD on a separate PR once I have that set-up.

[akien-mga]

Not a fan of having to import sys directly to specifically detect FreeBSD. Any other solution would be appreciated.

Yeah it's a bit hacky and would break in cross-compilation (if there's any cross-compilation toolchain that targets FreeBSD).
But for now it's acceptable, *BSDs are supported on a best-effort basis so it's not unreasonable to expect users who want to target those platforms to build on the target platform.

[MBCX]

So for the thirdparty code, I kept the changes as Akien mentioned, but also created a diff file in the patches folder. Sounds good @adamscott?

[akien-mga]

Looks good to me.

[Repiteo]

Thanks!