Was getting error:
```
running: "sh" "/home/flubba86/rust/src/libstd/../libbacktrace/configure" "--with-pic" "--disable-multilib" "--disable-shared" "--disable-host-shared" "--host=asmjs-unknown-emscripten" "--build=x86_64-unknown-linux-gnu"
...
Invalid configuration `asmjs-unknown-emscripten': system `emscripten' not recognized
```
Undo change to libbacktrace configure script.
Modify libstd build.rs to not build libbacktrace in the case of targeting emscripten.
let target = env::var("TARGET").unwrap();
let host = env::var("HOST").unwrap();
- if !target.contains("apple") && !target.contains("msvc") {
+ if !target.contains("apple") && !target.contains("msvc") && !target.contains("emscripten"){
build_libbacktrace(&host, &target);
}