CMake: Refine CXX_STANDARD setting

- Set CXX_STANDARD_REQUIRED ON so that we error out early
  if CMake thinks that the compiler does not support the
  used standard.
- Set CXX_EXTENSIONS OFF so that we get less compiler
  specific behavior.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
(cherry picked from commit 9b8797fe5e)

CMakeLists.txt

@@ -21,6 +21,8 @@ set(CMAKE_LINKER_FLAGS_ASAN
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake
                       ${CMAKE_MODULE_PATH})
 set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_C_STANDARD 99)
 
 include(CTest)

cmake/findcoredeps.cmake

@@ -1,12 +1,11 @@
 cmake_minimum_required(VERSION 3.13...3.28)
 
 set(CMAKE_CXX_STANDARD 17)
-
-#cmake_policy(SET CMP0079 NEW)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
 
 set(CORE_DIR ${CMAKE_CURRENT_LIST_DIR}/..)
 
-
 set(DEP_DIR ${CORE_DIR}/../deps CACHE PATH "Dependencies")
 option(USE_MBEDTLS "Use mbed TLS instead of OpenSSL")