detect_family_inner may block indefinietely

[ChrisDenton]

This line can, in certain situations, block indefinitely

cc-rs/src/tool.rs

Lines 161 to 167 in a0441c3

	let accepts_cl_style_flags = run(Command::new(path).arg("-?"), path, &{
	// the errors are not errors!
	let mut cargo_output = cargo_output.clone();
	cargo_output.warnings = cargo_output.debug;
	cargo_output.output = OutputKind::Discard;
	cargo_output
	})

If cl.exe detects there's a console then it will paginate its output, prompting the user to press enter to continue. This is a problem when cargo_output.debug is set to true because then we will forward stderr to what may, potentially, be a console handle.

Easiest fix is probably to set .stdin to Stdio::null() so cl.exe always knows there's no input to wait for.