Hi,
I think it would be nice to export some definitions used by the library so external applications can access them. For example, if the library has been compiled with ZMQ support and trees can be monitored.
Something like this could be added at the end of CMakeLists.txt:

if( ZMQ_FOUND )
    target_compile_definitions(${BEHAVIOR_TREE_LIBRARY} PUBLIC ZMQ_FOUND)
endif()