cxx: fix build with CARGO_TARGET_DIR set

[fw-immunant]

Fixes #2335. This build.rs file is already ours rather than matching the upstream one, so I think further changes are fine--stop me if not!

[mgeisler]

Fixes #2335. This build.rs file is already ours rather than matching the upstream one, so I think further changes are fine--stop me if not!

Alright, I had honestly no idea what we had customized the build file 😄

Thanks a lot for improving it, I only think we should add a comment explaining what this does.

[mgeisler]

Suggested change

	let src_dir = {
	// Handle $CARGO_TARGET_DIR being set.
	let src_dir = {

[mgeisler]

Looking at this, you could consider writing it more directly as

// Handle $CARGO_TARGET_DIR being set.
let mut src_dir = std::env::var_os("CARGO_TARGET_DIR").unwrap_or("../../../target".into());
src_dir.push("/cxxbridge/demo/src");

Also, do you know how this relates to the $OUT_DIR variable which Cargo sets?

[fw-immunant]

Done.

As I understand it, $OUT_DIR refers to a temporary directory, and is an output of Cargo; $CARGO_TARGET_DIR is an input to Cargo and might change what $OUT_DIR ends up being (but I'm not sure, I don't touch build scripts often... unless they break).