message: make Python binding generation conditional on presence of pybind11 submodule
diff: @@ -7,6 +7,11 @@ catkin_simple()
  set(CMAKE_MACOSX_RPATH 0)
  add_definitions(-std=c++11 -Wno-sign-compare -Wno-unused-value)
  +if (EXISTS "${CMAKE_CURRENT_LIST_DIR}/pybind11/CMakeLists.txt")
  + set(HAVE_PYBIND11 TRUE)
  + message(STATUS "Found pybind11; generating Python bindings")
  +endif()
  +
  ############
  # PROTOBUF #
  ############
  @@ -67,6 +72,7 @@ target_link_libraries(${PROJECT_NAME} ${PROTOBUF_LIBRARIES})
  ###################
  # PYTHON BINDINGS #
  ###################
  +if(HAVE_PYBIND11)
  catkin_python_setup()
  # TODO(mereweth@jpl.nasa.gov) - how to tell pybind11 about an existing library?
  @@ -87,6 +93,7 @@ target_link_libraries(voxbloxpy PUBLIC ${PROTOBUF_LIBRARIES} ${catkin_LIBRARIES}
  set_target_properties(voxbloxpy
  PROPERTIES LIBRARY_OUTPUT_DIRECTORY
  ${CATKIN_DEVEL_PREFIX}/${CATKIN_PACKAGE_PYTHON_DESTINATION}/../)
  +endif()
  ############
  # BINARIES #
  