rust 1.15 regression in cargo CARGO_OUT handling

[brson]

Linking this here for the regression tag.

rust-lang/cargo#3368

https://internals.rust-lang.org/t/regression-report-stable-2016-11-08-vs-nightly-2016-12-06/4462/3

[alexcrichton]

Checking back in on this, there haven't been any more comments in rust-lang/cargo#3368. For the breakage we've seen:

• pinyin has a PR - Use env::var_os intead of env! in build script mozillazg/rust-pinyin#5
• rasen is already fixed by removing the build script - leops/rasen@fa6ee92
• zip_codes has a PR - Use env::var_os intead of env! in build script SkylerLipthay/zip_codes#2

Still unsure what to do about this regression, but wanted to give a status report.

[brson]

I think we're pretty committed to leaving this as is.