esp-idf/components/mqtt/CMakeLists.txt

if(NOT EXISTS ${CMAKE_CURRENT_LIST_DIR}/esp-mqtt/CMakeLists.txt)
    set(COMMAND  "git submodule update --init --recursive")
    message(FATAL_ERROR "Missing esp-mqtt submodule. Please run '${COMMAND}' in ESP-IDF directory to fix this.")
else()
    include(${CMAKE_CURRENT_LIST_DIR}/esp-mqtt/CMakeLists.txt)
endif()