Github user andschwa commented on the issue:
https://github.com/apache/mesos/pull/247
@mpereira Oh I know what's going on:
> Please make sure Java proto files are generated in
../build/src/java/generated folder.
The site generator expects the build to have been run, at least to the
point of proto files being generated.
Try
```
mkdir build
cd build
cmake -DENABLE_JAVA=ON ..
cmake --build . --target mesos-protobufs
```
And then try generating the site again, I think it'll work.---
