lv_sim: cmake: let the user specify the C++ standard version

lv_sim/CMakeLists.txt

@@ -7,9 +7,17 @@ set(PROJECT_VERSION ${CMAKE_MATCH_1})
 message(STATUS "InfiniTime PROJECT_VERSION extracted: ${PROJECT_VERSION}")
 
 project(lv_sim VERSION ${PROJECT_VERSION} LANGUAGES C CXX)
+# https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html
+string(COMPARE EQUAL "${CMAKE_CXX_STANDARD}" "" no_cmake_cxx_standard_set)
+if(no_cmake_cxx_standard_set)
+    set(CMAKE_CXX_STANDARD 17)
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
+    set(CMAKE_CXX_EXTENSIONS OFF)
+    message(STATUS "Using default C++ standard ${CMAKE_CXX_STANDARD}")
+else()
+    message(STATUS "Using user specified C++ standard ${CMAKE_CXX_STANDARD}")
+endif()
 set(CMAKE_C_STANDARD 11)#C11
-set(CMAKE_CXX_STANDARD 17)#C17
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 file(GLOB_RECURSE INCLUDES "lv_drivers/*.h" "../src/libs/lvgl/src/*.h"  "./*.h" )
 file(GLOB_RECURSE SOURCES  "lv_drivers/*.c" "../src/libs/lvgl/src/*.c" )