Add feature for doc_cfg, specify allowed cfgs in build.rs

[Tiwalun]

Specify possible configs as described in https://doc.rust-lang.org/nightly/rustc/check-cfg.html, and fix compilation with --cfg docsrs.

[JohnAZoidberg]

Seems this is only supported in nightly :/

> cargo build
    Updating crates.io index
  Downloaded hidapi v2.6.2
  Downloaded 1 crate (128.1 KB) in 0.26s
   Compiling libusb1-sys v0.7.0
   Compiling hidapi v2.6.2
error: unsupported output in build script of `hidapi v2.6.2`: `cargo::rustc-check-cfg=cfg(hidapi)`
Found a `cargo::key=value` build directive which is reserved for future use.
Either change the directive to `cargo:key=value` syntax (note the single `:`) or upgrade your version of Rust.
See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script for more information about build script outputs.
warning: build failed, waiting for other jobs to finish...

> cargo --version
cargo 1.75.0 (1d8b05cdd 2023-11-20)

[Tiwalun]

I think this is related to the use of two colons in cargo::rustc-check-cfg=cfg(hidapi), if compatibility with versions older than 1.77 is required that has to be changed to cargo:rustc-check-cfg=cfg(hidapi). I don't think the actual rustc-check-cfg directive should cause a problem.

@ruabmbua Does this crate have any policies regarding which rust versions are supported?

[ruabmbua]

There is no explicit MSRV, but I try to keep it working for some (reasonably) recent packaged rustc versions from linux distros. Seems like debian stable does not have 1.77+ yet, so I will make a new release with a fix.

[ruabmbua]

Fixed in https://github.com/ruabmbua/hidapi-rs/releases/tag/v2.6.3