Fix querying emcc on windows

[guusw]

Pretty minor change, just queries emcc.bat since emcc will not work on windows

For example this simple script will output this on windows

fn main() {
    let output = std::process::Command::new("emcc.bat").arg("-dumpversion").output().ok();
    println!("{:?}", output);
    let output1 = std::process::Command::new("emcc").arg("-dumpversion").output().ok();
    println!("{:?}", output1);
}

rustc test.rs && ./test.exe
Some(Output { status: ExitStatus(ExitStatus(0)), stdout: "3.1.74\r\n", stderr: "" })
None

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @tgross35 (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[tgross35]

Nit: could you use if cfg!(target_os = "windows") { ... } here instead? cfg!(...) is slightly preferred over #[cfg(...)] where possible since it allows the code to be checked on all OSs.

Otherwise lgtm, please squash with the change.

Fyi @hoodmane.

[guusw]

Applied this

[guusw]

Could this go on 0.2 as well once it's merged?

[hoodmane]

Cc @messense maybe we want this change in maturin too?

[messense]

Cc @messense maybe we want this change in maturin too?

Done in PyO3/maturin#2478

[tgross35]

Could this go on 0.2 as well once it's merged?

Fyi anyone can add the label, it's just

@rustbot label +stable-nominated

[tgross35]

Looks like this needs a rebase, could you do that?

[tgross35]

@rustbot author for the rebase