VS: Do not fail on Windows 10 with VS 2015 if no SDK is available (#15929)

Since commit v3.4.0-rc1~5^2~1 (VS: Add support for selecting the Windows
10 SDK, 2015-09-30) the VS 2015 generator requires a Windows 10 SDK to
be available when CMAKE_SYSTEM_VERSION specifies Windows 10 (e.g.  when
building on a Windows 10 host).  Howewver, it is possible to install VS
2015 without any Windows 10 SDK.  Instead of failing with an error
message about the lack of a Windows 10 SDK, simply tolerate this case
and use the default Windows 8.1 SDK.  Since building for Windows Store
still requires the SDK, retain the diagnostic in that case.

Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst

@@ -8,4 +8,5 @@ specification of a target Windows version to select a corresponding SDK.
 The :variable:`CMAKE_SYSTEM_VERSION` variable may be set to specify a
 version.  Otherwise CMake computes a default version based on the Windows
 SDK versions available.  The chosen Windows target version number is provided
-in ``CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION``.
+in ``CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION``.  If no Windows 10 SDK
+is available this value will be empty.

Source/cmGlobalVisualStudio14Generator.cxx

@@ -115,7 +115,7 @@ bool cmGlobalVisualStudio14Generator::InitializeWindows(cmMakefile* mf)
 {
   if (cmHasLiteralPrefix(this->SystemVersion, "10.0"))
     {
-    return this->SelectWindows10SDK(mf);
+    return this->SelectWindows10SDK(mf, false);
     }
   return true;
 }
@@ -143,17 +143,18 @@ bool cmGlobalVisualStudio14Generator::InitializeWindowsStore(cmMakefile* mf)
     }
   if (cmHasLiteralPrefix(this->SystemVersion, "10.0"))
     {
-    return this->SelectWindows10SDK(mf);
+    return this->SelectWindows10SDK(mf, true);
     }
   return true;
 }
 
 //----------------------------------------------------------------------------
-bool cmGlobalVisualStudio14Generator::SelectWindows10SDK(cmMakefile* mf)
+bool cmGlobalVisualStudio14Generator::SelectWindows10SDK(cmMakefile* mf,
+                                                         bool required)
 {
   // Find the default version of the Windows 10 SDK.
   this->WindowsTargetPlatformVersion = this->GetWindows10SDKVersion();
-  if (this->WindowsTargetPlatformVersion.empty())
+  if (required && this->WindowsTargetPlatformVersion.empty())
     {
     std::ostringstream  e;
     e << "Could not find an appropriate version of the Windows 10 SDK"

Source/cmGlobalVisualStudio14Generator.h

@@ -39,7 +39,7 @@ protected:
   bool IsWindowsStoreToolsetInstalled() const;
 
   virtual const char* GetIDEVersion() { return "14.0"; }
-  virtual bool SelectWindows10SDK(cmMakefile* mf);
+  virtual bool SelectWindows10SDK(cmMakefile* mf, bool required);
 
   // Used to verify that the Desktop toolset for the current generator is
   // installed on the machine.