Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

)

`setns`, `preadv64v2`, and `pwritev64v2` were introduced more recently
than the earliest glibc we support, so use the `weak!` mechanism for
them, with fallbacks using `syscall`.

* Fix `use-libc-auxv` to use a weak dependency on `getauxval`.

glibc <= 2.15 lacks `getauxval`, so avoid a strong dependency on it in
the `use-libc-auxv` implementation.

FreeBSD CI has been broken for some time with this error

```
[4/4] Extracting curl-7.87.0_1: .......... done
curl https://sh.rustup.rs -sSf --output rustup.sh
ld-elf.so.1: /usr/local/lib/libcurl.so.4: Undefined symbol "nghttp2_option_set_no_rfc9113_leading_and_trailing_ws_validation"
```

Comment it out for now.

The issue was using snapshots of already released versions, which
is not supported.

* Add more `cargo:rerun-if-env-changed`'s to build.rs

Add more `cargo:rerun-if-env-changed`'s to build.rs for rustix's
features, configuration flags, and the toolchain variables rustix uses
for autoconfiguration, to hopefully ensure it gets rerun when anything
changes.

See also #526.

* Don't include all the cargo features.