COMP: Ensure that C++ standard version is at least C++17

Aims to avoid compile errors from macosx-10.9-universal2-3.11/Xcode_13.2.1
MacOSX12.1.sdk/AppleClang 13.0.0.13000029 saying:

> error: no template named 'is_integral_v' in namespace 'std'; did you mean 'is_integral'?

CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.16.3)
 project(Elastix)
 
-if(NOT CMAKE_CXX_STANDARD)
+if(NOT CMAKE_CXX_STANDARD OR CMAKE_CXX_STANDARD LESS 17)
   # SuperElastix/elastix uses C++17 specific features in some of its header files.
   set(CMAKE_CXX_STANDARD 17)
 endif()