There are different implementations and signatures for `get_pollfd` depending
on whether the target platform is unix or windows. When generating the doc,
we need both implementations to appear regardless of the target platform. This
commit is inspired by the way Rust `std` library deals with `process::Command`
OS dependent variants
(https://doc.rust-lang.org/std/process/struct.Command.html#impl-CommandExt).

Documentation can't be accurate though as we can't use the`std::os::windows`
on `unix` and vice versa. As a workaround a fake fd class matching the other
platform is declared.

This could be further enhanced once `#[doc(cfg(...))]` is stabilized
(https://github.com/rust-lang/rust/issues/43781) by declaring `#[doc(cfg(unix))]`
or `#[doc(cfg(windows))]` instead of the hard coded comments `This is supported
on **Windows/Unix** only`. Unfortunately, these comments disappear when
generating will `--all-features` because they are not part of the documentation
in the gir file.

Fixes https://github.com/sdroege/gstreamer-rs/pull/93

When feature `dox` is selected but not `futures`, the `futures` crate is not
available as a dependency leading to a "can't find crate for `futures`" error.

Fixes https://github.com/sdroege/gstreamer-rs/pull/92

This should improve logging performance if the threshold is not high
enough.