VS: Change CMAKE_VS_WINRT_BY_DEFAULT to not implicitly enable WinRT

Original behaviour would unconditionally enable WinRT for all projects
so source file flag generation code can acknowledge WinRT being present
and disable it for C language source files.  An unintentional result of
that approach is that WinRT is enabled for ALL projects, including C++
projects/source files with no way to disable it

Instead use `CMAKE_VS_WINRT_BY_DEFAULT` as a hint that the platform is
WinRT-by-default and set global `CompileAsWinRT` flag to `false` unless
it was explicitly requested by either `WINRT_COMPONENT` option or `/ZW`
compilation option - similar to what Windows Phone/Windows Store
platform logic does

In case WinRT compilation is enabled for a project by either of
aforementioned methods, C language source file override logic will still
kick in and disable CompileAsWinRT for C source files

Fixes: #20063

Help/variable/CMAKE_VS_WINRT_BY_DEFAULT.rst

@@ -1,8 +1,18 @@
 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``).
+Inform :ref:`Visual Studio Generators` for VS 2010 and above that the
+target platform enables WinRT compilation by default and it needs to
+be explicitly disabled if ``/ZW`` or :prop_tgt:`VS_WINRT_COMPONENT` is
+omitted (as opposed to enabling it when either of those options is
+present)
+
+This makes cmake configuration consistent in terms of WinRT among
+platforms - if you did not enable the WinRT compilation explicitly, it
+will be disabled (by either not enabling it or explicitly disabling it)
+
+Note: WinRT compilation is always explicitly disabled for C language
+source files, even if it is expliclty enabled for a project
 
 This variable is meant to be set by a
 :variable:`toolchain file <CMAKE_TOOLCHAIN_FILE>` for such platforms.

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2832,10 +2832,8 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
   }
 
   if (this->MSTools) {
-    // 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")) {
+    // If we have the VS_WINRT_COMPONENT set then force Compile as WinRT
+    if (this->GeneratorTarget->GetPropertyAsBool("VS_WINRT_COMPONENT")) {
       clOptions.AddFlag("CompileAsWinRT", "true");
       // For WinRT components, add the _WINRT_DLL define to produce a lib
       if (this->GeneratorTarget->GetType() == cmStateEnums::SHARED_LIBRARY ||
@@ -2843,7 +2841,8 @@ bool cmVisualStudio10TargetGenerator::ComputeClOptions(
         clOptions.AddDefine("_WINRT_DLL");
       }
     } else if (this->GlobalGenerator->TargetsWindowsStore() ||
-               this->GlobalGenerator->TargetsWindowsPhone()) {
+               this->GlobalGenerator->TargetsWindowsPhone() ||
+               this->Makefile->IsOn("CMAKE_VS_WINRT_BY_DEFAULT")) {
       if (!clOptions.IsWinRt()) {
         clOptions.AddFlag("CompileAsWinRT", "false");
       }

Tests/RunCMake/VS10Project/VsWinRTByDefault-check.cmake

@@ -57,9 +57,9 @@ macro(checkCompileAsWinRT projectPath)
   endif()
 endmacro()
 
-checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagOnlyC.vcxproj" GLOBAL true OVERRIDES_DISABLE empty.c)
-checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagMixedCAndCxx.vcxproj" GLOBAL true OVERRIDES_DISABLE empty.c)
-checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagOnlyCxx.vcxproj" GLOBAL true)
+checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagOnlyC.vcxproj" GLOBAL false)
+checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagMixedCAndCxx.vcxproj" GLOBAL false)
+checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/noFlagOnlyCxx.vcxproj" GLOBAL false)
 
 checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/flagOnlyC.vcxproj" GLOBAL true OVERRIDES_DISABLE empty.c)
 checkCompileAsWinRT("${RunCMake_TEST_BINARY_DIR}/flagMixedCAndCxx.vcxproj" GLOBAL true OVERRIDES_DISABLE empty.c)