git subrepo clone --branch=20.41.6 git@github.com:ETLCPP/etl.git components/etl

subrepo:
  subdir:   "components/etl"
  merged:   "be5537ec"
upstream:
  origin:   "git@github.com:ETLCPP/etl.git"
  branch:   "20.41.6"
  commit:   "be5537ec"
git-subrepo:
  version:  "0.4.9"
  origin:   "???"
  commit:   "???"

components/etl/CMakeLists.txt

@@ -0,0 +1,95 @@
+#######################################################################
+# The Embedded Template Library (https://www.etlcpp.com/)
+#######################################################################
+cmake_minimum_required(VERSION 3.10)
+
+include(${CMAKE_CURRENT_SOURCE_DIR}/cmake/helpers.cmake)
+
+set(MSG_PREFIX "etl |")
+etl_determine_version_with_git(${GIT_DIR_LOOKUP_POLICY})
+if(NOT ETL_VERSION)
+    etl_determine_version_with_file("version.txt")
+endif()
+
+project(etl VERSION ${ETL_VERSION} LANGUAGES CXX)
+
+option(BUILD_TESTS "Build unit tests" OFF)
+option(NO_STL "No STL" OFF)
+# There is a bug on old gcc versions for some targets that causes all system headers
+# to be implicitly wrapped with 'extern "C"'
+# Users can add set(NO_SYSTEM_INCLUDE ON) to their top level CMakeLists.txt to work around this.
+option(NO_SYSTEM_INCLUDE "Do not include with -isystem" OFF)
+if (NO_SYSTEM_INCLUDE)
+    set(INCLUDE_SPECIFIER "")
+else()
+    set(INCLUDE_SPECIFIER "SYSTEM")
+endif()
+
+add_library(${PROJECT_NAME} INTERFACE)
+# This allows users which use the add_subdirectory or FetchContent
+# to use the same target as users which use find_package
+add_library(etl::etl ALIAS ${PROJECT_NAME})
+
+include(GNUInstallDirs)
+
+target_include_directories(${PROJECT_NAME} ${INCLUDE_SPECIFIER} INTERFACE
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+        $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+        )
+
+target_link_libraries(${PROJECT_NAME} INTERFACE)
+
+# only install if top level project
+if(${CMAKE_PROJECT_NAME} STREQUAL ${PROJECT_NAME})
+    # Steps here based on excellent guide: https://dominikberner.ch/cmake-interface-lib/
+    # Which also details all steps
+    include(CMakePackageConfigHelpers)
+    install(TARGETS ${PROJECT_NAME}
+        EXPORT ${PROJECT_NAME}Targets
+        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+    )
+    if(ETL_VERSION)
+        # Generate the package configuration files using CMake provided macros
+        if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.14)
+            # Generate the package configuration files using CMake provided macros
+            write_basic_package_version_file(
+                "${PROJECT_NAME}ConfigVersion.cmake"
+                COMPATIBILITY SameMajorVersion
+                ARCH_INDEPENDENT
+            )
+        else()
+            # This is needed for CMake < 3.14
+            # because the ARCH_INDEPENDENT option is not available
+            # in CMake < 3.14
+            # See https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html
+            # for more information
+            write_basic_package_version_file(
+                "${PROJECT_NAME}ConfigVersion.cmake"
+                COMPATIBILITY SameMajorVersion
+            )
+        endif()
+    endif()
+    configure_package_config_file(
+        "${PROJECT_SOURCE_DIR}/cmake/${PROJECT_NAME}Config.cmake.in"
+        "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
+        INSTALL_DESTINATION
+        ${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake)
+
+    # Install target file, then package configuration files, and finally the headers
+    install(EXPORT ${PROJECT_NAME}Targets
+        FILE ${PROJECT_NAME}Targets.cmake
+        NAMESPACE ${PROJECT_NAME}::
+        DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake)
+    install(FILES "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
+        "${PROJECT_BINARY_DIR}/${PROJECT_NAME}ConfigVersion.cmake"
+        DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake)
+    install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/etl DESTINATION include)
+
+endif()
+
+if (BUILD_TESTS)
+    enable_testing()
+    add_subdirectory(test)
+endif()