Fetch the host triple from rustc instead of using a static value

[laumann]

Fixes #12

[laumann]

@Ryman Do you think this solution will work?

[Ryman]

I'm unsure, this would work for my case, but probably doesn't help for people doing cross-compiles.

I thought Cargo sets env vars, but it seems it only does it for build scripts. Perhaps compiletest could use a buildscript to grab these from cargo and then load them via include!(concat!(env!("OUT_DIR"), "/targets.rs")); within compiletest itself (hidden form usercode).

Maybe keeping it simple is best to start with, but you might want to get an opinion from someone more knowledgeable on the build script story!

[laumann]

Ok, I think we'll deal with requirements for cross-compiles as they arise and I'll keep the build script idea in mind. Thanks for the input 😄