Fix Android clang compiler path when building with Windows host

[Osspial]

On Windows, the Android NDK provides the %ANDROID_TARGET%-clang executable as a .cmd file rather than an .exe file. std::process::Command can only resolve .exe commands when the command extension isn't explicitly provided (see rust-lang/rust#37519 (comment)), so cc-rs mistakenly thinks the %ANDROID_TARGET%-clang compiler isn't installed on the system even when it's in the path. This PR fixes that.

[alexcrichton]

To confirm, this has always been the case, right? Should we perhaps check for .exe and fall back to .cmd if it's available?

[Osspial]

As far as I understand this has always been the case, but there's no harm in checking for the .exe file. I've updated the code to do so.

[alexcrichton]

Thanks! Actually though while you're at it, this is something that I missed previously. Could you replace .spawn() with .output()? Otherwise I think this runs a risk of creating a zombie process.

[alexcrichton]

Ah it's ok I'll tweak that locally

[Osspial]

It looks like 4469060 changed the check at lib.rs:1994 from is_err to is_ok, which broke this fix.

[alexcrichton]

Oops sorry about that! I mistakenly assumed which compiler was being tested. I pushed up a new release which I think should fix this.

[Osspial]

Looks like that's fixed things. Thanks!