VS: Add option to tell generator that platfrom is WinRT by default

Create a ``CMAKE_VS_WINRT_BY_DEFAULT`` variable to indicate this.

Fixes: #18286

Help/manual/cmake-variables.7.rst

@@ -401,6 +401,7 @@ Variables that Control the Build
    /variable/CMAKE_VS_SDK_LIBRARY_WINRT_DIRECTORIES
    /variable/CMAKE_VS_SDK_REFERENCE_DIRECTORIES
    /variable/CMAKE_VS_SDK_SOURCE_DIRECTORIES
+   /variable/CMAKE_VS_WINRT_BY_DEFAULT
    /variable/CMAKE_WIN32_EXECUTABLE
    /variable/CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS
    /variable/CMAKE_XCODE_ATTRIBUTE_an-attribute

Help/variable/CMAKE_VS_WINRT_BY_DEFAULT.rst

@@ -0,0 +1,8 @@
+CMAKE_VS_WINRT_BY_DEFAULT
+-------------------------
+
+Tell :ref:`Visual Studio Generators` for VS 2010 and above that the
+target platform compiles as WinRT by default (compiles with ``/ZW``).
+
+This variable is meant to be set by a
+:variable:`toolchain file <CMAKE_TOOLCHAIN_FILE>` for such platforms.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2571,8 +2571,10 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
   }
 
   if (this->MSTools) {
-    // If we have the VS_WINRT_COMPONENT set then force Compile as WinRT.
-    if (this->GeneratorTarget->GetPropertyAsBool("VS_WINRT_COMPONENT")) {
+    // If we have the VS_WINRT_COMPONENT or CMAKE_VS_WINRT_BY_DEFAULT
+    // set then force Compile as WinRT.
+    if (this->GeneratorTarget->GetPropertyAsBool("VS_WINRT_COMPONENT") ||
+        this->Makefile->IsOn("CMAKE_VS_WINRT_BY_DEFAULT")) {
       clOptions.AddFlag("CompileAsWinRT", "true");
       // For WinRT components, add the _WINRT_DLL define to produce a lib
       if (this->GeneratorTarget->GetType() == cmStateEnums::SHARED_LIBRARY ||