VS: Add option for custom Win10 SDK version maximum

Since commit 83ddc4d289 (VS: Do not select a Windows SDK too high for
current VS version, 2017-08-07, v3.13.0-rc1~72^2~2) we enforce a maximum
SDK version for the VS 2015 generator.  The blog post linked in the
original commit is no longer available, but it can be seen here:

* https://web.archive.org/web/20190108032520/https://blogs.msdn.microsoft.com/chuckw/2018/10/02/windows-10-october-2018-update/

In particular, it states:

> VS 2015 Users: The Windows 10 SDK (15063, 16299, 17134, 17763)
> is officially only supported for VS 2017.

However, in some circumstances a higher version can be used.

Add a `CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM` to override the
generator's default maximum SDK version.

Fixes: #20633

1 files changed, 26 insertions, 5 deletions

diff --git a/Source/cmGlobalVisualStudio14Generator.cxx b/Source/cmGlobalVisualStudio14Generator.cxx
index 451d448..aa1b7ad 100644
--- a/Source/cmGlobalVisualStudio14Generator.cxx
+++ b/Source/cmGlobalVisualStudio14Generator.cxx
@@ -169,7 +169,8 @@ bool cmGlobalVisualStudio14Generator::SelectWindows10SDK(cmMakefile* mf,
                                                          bool required)
 {
   // Find the default version of the Windows 10 SDK.
-  std::string const version = this->GetWindows10SDKVersion();
+  std::string const version = this->GetWindows10SDKVersion(mf);
+
   if (required && version.empty()) {
     std::ostringstream e;
     e << "Could not find an appropriate version of the Windows 10 SDK"
@@ -233,8 +234,25 @@ bool cmGlobalVisualStudio14Generator::IsWindowsStoreToolsetInstalled() const
                                           cmSystemTools::KeyWOW64_32);
 }
 
-std::string cmGlobalVisualStudio14Generator::GetWindows10SDKMaxVersion() const
+std::string cmGlobalVisualStudio14Generator::GetWindows10SDKMaxVersion(
+  cmMakefile* mf) const
 {
+  // if the given value is set, it can either be OFF/FALSE or a valid SDK
+  // string
+  if (std::string const* value =
+        mf->GetDef("CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION_MAXIMUM")) {
+
+    // If the value is some off/false value, then there is NO maximum set.
+    if (cmIsOff(value)) {
+      return std::string();
+    }
+    // If the value is something else, trust that it is a valid SDK value.
+    else if (value) {
+      return *value;
+    }
+    // If value is an invalid pointer, leave result unchanged.
+  }
+
   // The last Windows 10 SDK version that VS 2015 can target is 10.0.14393.0.
   //
   // "VS 2015 Users: The Windows 10 SDK (15063, 16299, 17134, 17763) is
@@ -267,7 +285,8 @@ public:
 };
 #endif
 
-std::string cmGlobalVisualStudio14Generator::GetWindows10SDKVersion()
+std::string cmGlobalVisualStudio14Generator::GetWindows10SDKVersion(
+  cmMakefile* mf)
 {
 #if defined(_WIN32) && !defined(__CYGWIN__)
   std::vector<std::string> win10Roots;
@@ -317,8 +336,10 @@ std::string cmGlobalVisualStudio14Generator::GetWindows10SDKVersion()
     i = cmSystemTools::GetFilenameName(i);
   }
 
-  // Skip SDKs that cannot be used with our toolset.
-  std::string maxVersion = this->GetWindows10SDKMaxVersion();
+  // Skip SDKs that cannot be used with our toolset, unless the user does not
+  // want to limit the highest supported SDK according to the Microsoft
+  // documentation.
+  std::string maxVersion = this->GetWindows10SDKMaxVersion(mf);
   if (!maxVersion.empty()) {
     cm::erase_if(sdks, WindowsSDKTooRecent(maxVersion));
   }