Using compile cache can break linking certain library builds

When compiling a Build.xml like this, the build fails on mac while linking the default target with `HXCPP_COMPILE_CACHE` enabled:

```xml

```

```
fatal error: /Applications/Xcode_15.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file list file: obj/darwin64/all_objs is empty
```

The problem is that the `lib` files are cached by hxcpp in a static library, and apart from that there are no object files. This means there is an empty `all_objs` file generated, which libtool doesn't seem to like. On other platforms (with different static linking tools) this is not an issue.