Get fast_lio cmake execute the git submodule init

patches/fast_lio.patch

@@ -1,14 +1,37 @@
 diff --git a/CMakeLists.txt b/CMakeLists.txt
-index be3040b..4931c35 100644
+index be3040b..3176847 100644
 --- a/CMakeLists.txt
 +++ b/CMakeLists.txt
-@@ -5,17 +5,16 @@ if(NOT CMAKE_BUILD_TYPE)
+@@ -5,17 +5,39 @@ if(NOT CMAKE_BUILD_TYPE)
    set(CMAKE_BUILD_TYPE Release)
  endif()
  
 -ADD_COMPILE_OPTIONS(-std=c++14)
 -ADD_COMPILE_OPTIONS(-std=c++14)
 -set(CMAKE_CXX_FLAGS "-std=c++14 -O3")
++# Ensure the bundled ikd-Tree submodule (include/ikd-Tree/ikd_Tree.cpp) is present.
++# Auto-initialize it when building from a git checkout; otherwise fail with guidance.
++if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/include/ikd-Tree/ikd_Tree.cpp")
++  find_package(Git QUIET)
++  if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
++    message(STATUS "fast_lio: initializing git submodule include/ikd-Tree")
++    execute_process(
++      COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
++      WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
++      RESULT_VARIABLE GIT_SUBMOD_RESULT)
++    if(NOT GIT_SUBMOD_RESULT EQUAL "0")
++      message(FATAL_ERROR
++        "git submodule update --init failed with ${GIT_SUBMOD_RESULT}. "
++        "Run it manually in ${CMAKE_CURRENT_SOURCE_DIR}.")
++    endif()
++  endif()
++  if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/include/ikd-Tree/ikd_Tree.cpp")
++    message(FATAL_ERROR
++      "include/ikd-Tree/ikd_Tree.cpp is missing and could not be fetched automatically "
++      "(no git checkout available). Run: git submodule update --init --recursive")
++  endif()
++endif()
++
 +ADD_COMPILE_OPTIONS(-std=c++17)
 +set(CMAKE_CXX_FLAGS "-std=c++17 -O3")