Don't vendor protoc on Windows

[XAMPPRocky]

protobuf-src depends on auto tools which requires sh, so this means if you're trying to compile on Windows, it will fail.

error: failed to run custom build command for `protobuf-src v1.1.0+21.5`

Caused by:
  process didn't exit successfully: `C:\Users\erin.power\quilkin\target\release\build\protobuf-src-668a2e07a68a77fb\build-script-build` (exit code: 101)
  --- stderr
  thread 'main' panicked at '
  `sh` is required to run `configure`

  build script failed, must exit now', C:\Users\erin.power\.cargo\registry\src\index.crates.io-6f17d22bba15001f\autotools-0.2.6\src\lib.rs:781:5
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

[markmandel]

MaterializeInc/rust-protobuf-native#4 is relevant, with links off to how some other projects solved the issue.

[benesch]

The latest version of protobuf-src now builds with CMake rather than autotools. I've not yet tested this, but it may "just work" with Windows now.