winrt: build with c++20 by default

Author: axxel

wrappers/winrt/CMakeLists.txt

@@ -15,6 +15,12 @@ set (CMAKE_CONFIGURATION_TYPES "RelWithDebInfo;Release" CACHE STRING "" FORCE)
 set (WINRT ON)
 set (CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} /INCREMENTAL:NO" )
 
+if (NOT DEFINED CMAKE_CXX_STANDARD)
+    set (CMAKE_CXX_STANDARD 20)
+    # Allow the fallback to earlier versions if the compiler does not support it.
+    set(CMAKE_CXX_STANDARD_REQUIRED OFF)
+endif()
+
 add_definitions (-DUNICODE -D_UNICODE)
 
 if (MSVC)