Cross-Compiling for ARMv7

If I try cross-compiling a test-application I get following error (cross compiling a simple println!("Hello World") works, saying the error comes by trying to compile a webview example):

```
error: failed to run custom build command for `webview-sys v0.1.0`
process didn't exit successfully: `/home/user/TestApp/target/debug/build/webview-sys-08b7b43d331ebca0/build-script-build` (exit code: 101)
--- stderr
fatal: not a git repository (or any of the parent directories): .git
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: CrossCompilation', libcore/result.rs:945:5
```