Musl target builds

Hey there! Thanks for your awesome work, guys!

However, I have an issue building the crate for a linux-musl target while building any of the gfx-rs projects.
Here is the error message:
```
error: failed to run custom build command for `x11 v2.18.1`
process didn't exit successfully: `/home/fox/Documents/Development/Temp/gfx/target/release/build/x11-e03f3f938e7478ea/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: CrossCompilation', src/libcore/result.rs:997:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

warning: build failed, waiting for other jobs to finish...
error: build failed
```

Is there anything that can be done on my side?

Thanks!