-options argument is always loaded relative to Build.xml

When running:
```
haxelib run hxcpp ./path/Build.xml -options ./Options.txt
```

it would make sense for `Options.txt` to be loaded from the current directory, not from `./path/Options.txt`. Hxcpp currently always resolves it relative to the Build.xml directory.

It would also be nice to see an error if a manually specified Options file doesn't exist. Right now it is just ignored silently.