Cannot compile with Swift 4.0 on Ubuntu

```
<unknown>:0: error: unexpected 'commands' value (expected map)
<unknown>:0: error: unable to load build file
error: terminated(1): /home/martin/swift/usr/bin/swift-build-tool -f /home/martin/Downloads/CGtk-Linux-master/.build/debug.yaml main
```

```
Swift version 4.0 (swift-4.0-RELEASE)
Target: x86_64-unknown-linux-gnu
```