first impl. but wrong.

external/json/CMakeLists.txt

@@ -0,0 +1,41 @@
+cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
+
+##
+## PROJECT
+## name and version
+##
+project(nlohmann_json VERSION 3.10.4 LANGUAGES CXX)
+
+
+##
+## OPTIONS
+##
+option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
+
+##
+## CONFIGURATION
+##
+set(NLOHMANN_JSON_TARGET_NAME               ${PROJECT_NAME})
+
+if (JSON_MultipleHeaders)
+    set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/include/")
+    message(STATUS "Using the multi-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
+else()
+    set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/single_include/")
+    message(STATUS "Using the single-header code from ${NLOHMANN_JSON_INCLUDE_BUILD_DIR}")
+endif()
+
+##
+## TARGET
+## create target and add include path
+##
+add_library(${NLOHMANN_JSON_TARGET_NAME} INTERFACE)
+add_library(${PROJECT_NAME}::${NLOHMANN_JSON_TARGET_NAME} ALIAS ${NLOHMANN_JSON_TARGET_NAME})
+if (${CMAKE_VERSION} VERSION_LESS "3.8.0")
+    target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_range_for)
+else()
+    target_compile_features(${NLOHMANN_JSON_TARGET_NAME} INTERFACE cxx_std_11)
+endif()
+
+target_include_directories(${NLOHMANN_JSON_TARGET_NAME} INTERFACE ${NLOHMANN_JSON_INCLUDE_BUILD_DIR})
+