build(macos): Use C++17

To address:

In file included from src/itkWASMImageIOBase.cxx:18:
In file included from include/itkWASMImageIOBase.h:23:
In file included from Modules/IO/ImageBase/include/itkImageIOBase.h:30:
In file included from Modules/Core/Common/include/itkSymmetricSecondRankTensor.h:29:
In file included from Modules/Core/Common/include/itkSymmetricEigenAnalysis.h:1104:
In file included from Modules/Core/Common/include/itkSymmetricEigenAnalysis.hxx:22:
Modules/Core/Common/include/itkMakeUniqueForOverwrite.h:76:14: error: no member named 'make_unique_for_overwrite' in namespace 'std'

We could revisit after the GitHub Action Hosted Runner is updated from
macos-11 to macos-12. Windows needs 20 to avoid compile errors.

CMakeLists.txt

@@ -1,7 +1,11 @@
 cmake_minimum_required(VERSION 3.16.3)
 project(WebAssemblyInterface)
 
-set(CMAKE_CXX_STANDARD 20)
+if(WIN32)
+  set(CMAKE_CXX_STANDARD 20)
+else()
+  set(CMAKE_CXX_STANDARD 17)
+endif()
 
 set(WebAssemblyInterface_LIBRARIES WebAssemblyInterface)