Support Windows-GNU builds

[kirawi]

Right now, we've hardcoded MSVC:

helix/helix-term/build.rs

Lines 19 to 57 in cbbeca6

	pub(crate) fn link_icon_in_windows_exe(icon_path: &str) {
	let rc_exe = find_rc_exe().expect("Windows SDK is to be installed along with MSVC");
	let output = env::var("OUT_DIR").expect("Env var OUT_DIR should have been set by compiler");
	let output_dir = PathBuf::from(output);
	let rc_path = output_dir.join("resource.rc");
	write_resource_file(&rc_path, icon_path).unwrap();
	let resource_file = PathBuf::from(&output_dir).join("resource.lib");
	compile_with_toolkit_msvc(rc_exe, resource_file, rc_path);
	println!("cargo:rustc-link-search=native={}", output_dir.display());
	println!("cargo:rustc-link-lib=dylib=resource");
	}
	fn compile_with_toolkit_msvc(rc_exe: PathBuf, output: PathBuf, input: PathBuf) {
	let mut command = process::Command::new(rc_exe);
	let command = command.arg(format!(
	"/I{}",
	env::var("CARGO_MANIFEST_DIR")
	.expect("CARGO_MANIFEST_DIR should have been set by Cargo")
	));
	let status = command
	.arg(format!("/fo{}", output.display()))
	.arg(format!("{}", input.display()))
	.output()
	.unwrap();
	println!(
	"RC Output:\n{}\n------",
	String::from_utf8_lossy(&status.stdout)
	);
	println!(
	"RC Error:\n{}\n------",
	String::from_utf8_lossy(&status.stderr)
	);
	}

We should gracefully handle the case where MSVC is not included, or support GNU.

[kirawi]

I think it would be fine to just add #[cfg(target_env = "msvc")] and implement a variant of the code based on windres