Fix cross compilation on `stable-x86_64-pc-windows-gnu`

[adumbidiot]

Songbird version: 0.1.6

Rust version (rustc -V): rustc 1.52.1 (9bc8c42bb 2021-05-09)

Serenity/Twilight version: (version)

Output of ffmpeg -version, youtube-dl --version (if relevant): Not Relavent

Description: I was trying to work on compiling a serenity bot with the x86_64-pc-windows-gnu target but got stuck on audiopus-sys failing to compile, I think due to handling platform features incorrectly.

I managed to fix this by updating audiopus to 0.3.0-rc.0 in songbird. I can open a PR if it is wanted. The changes are breaking however as cmake is now a required dependency of audiopus.

~Related serenity issue: https://github.com/serenity-rs/serenity/issues/1001.~ It looks like that user tried to build on the windows gnu toolchain instead. I tried to replicate the issue, but got a different error than they did. However, updating audiopus seems to fix this issue as well.

Steps to reproduce:

1. Clone the current branch of this repository on Windows 10 with the toolchain and msys2 installed, with the appropriate compiler tools installed and available through the PATH.
2. Run cargo test --all-features --target x86_64-pc-windows-gnu
3. Wait for the compilation to fail:

PS D:\Github\songbird> cargo test --all-features --target x86_64-pc-windows-gnu
    Updating crates.io index
   Compiling songbird v0.1.6 (D:\Github\songbird)
   Compiling term v0.4.6
   Compiling native-tls v0.2.7
   Compiling mio v0.7.11
   Compiling audiopus_sys v0.1.8
   Compiling tracing v0.1.26
   Compiling rustls v0.19.1
   Compiling rustls v0.18.1
   Compiling poly1305 v0.6.2
   Compiling salsa20 v0.7.2
   Compiling mio v0.6.23
   Compiling audiopus v0.2.0
error: could not find native static library `opus`, perhaps an -L flag is missing?

error: aborting due to previous error

error: could not compile `audiopus_sys`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

[Lakelezz]

Yes, the RC of audiopus should fix cross-compiling. I wanted to port songbird to it myself but did not get around to it. Glad to see that you have, once songbird would work fine with it, I can release a non-RC.

[kangalio]

Songbird 0.3.0 now depends on audiopus 0.3.0-rc.0, so this issue can be closed, right?

[FelixMcFelix]

It should be yes, thanks for the reminder!