Make Java a required dependency

webodf.js needs it for closure compiler, and there is not much in the repo
which really makes sense to build without also building webodf.js

Played also with `project (WebODF C CXX Java)`, but that gave me
```
CMake Error: your Java compiler: "CMAKE_Java_COMPILER-NOTFOUND" was not found.   Please set CMAKE_Java_COMPILER to a valid compiler path or name.
```
which seemed strange and I could not find a quick solution to that, so left for future investigations.