If xeus and its dependencies are installed in non standard location, xeus-cpp needs to be aware of the related include directories to be able to process xcpp extensions correctly.

I think this needs to be addressed as it was done for xeus-cling

1. https://github.com/jupyter-xeus/xeus-cling/blob/93d61b98518c494fc0e195844c392fc0d40dd0d2/src/xinterpreter.cpp#L435
2. https://github.com/jupyter-xeus/xeus-cling/blob/93d61b98518c494fc0e195844c392fc0d40dd0d2/CMakeLists.txt#L332-L333