Git was called in the build directory and not the source directory.
This works fine when the build directory resides within the source
repository, but doesn't work for out-of-source builds. Example:

```
~/zigbuild$ cmake ../zig
fatal: not a git repository (or any of the parent directories): .git
Configuring zig version 0.2.0+
```

Use Git's `-C <path>` flag to always point to the source directory so
that it doesn't matter where the build directory lives.