VS: Add an environment variable for the Windows 10 kits directory

Define a `CMAKE_WINDOWS_KITS_10_DIR` environment variable to allow
users to tell CMake about a custom Windows 10 SDK directory.  We
choose to make this an environment variable rather than a CMake
variable or cache entry because:

* Using a custom directory also requires custom external MSBuild
  configuration.  Therefore users are already configuring a
  custom environment.

* The custom directory must be set consistently in all parts of
  a build including nested projects.  An environment variable
  avoids requiring users to thread the setting into nested builds.

Fixes: #16743

1 files changed, 8 insertions, 0 deletions

diff --git a/Source/cmGlobalVisualStudio14Generator.cxx b/Source/cmGlobalVisualStudio14Generator.cxx
index 4602d93..df086d3 100644
--- a/Source/cmGlobalVisualStudio14Generator.cxx
+++ b/Source/cmGlobalVisualStudio14Generator.cxx
@@ -220,6 +220,14 @@ std::string cmGlobalVisualStudio14Generator::GetWindows10SDKVersion()
   std::vector<std::string> win10Roots;
 
   {
+    std::string win10Root;
+    if (cmSystemTools::GetEnv("CMAKE_WINDOWS_KITS_10_DIR", win10Root)) {
+      cmSystemTools::ConvertToUnixSlashes(win10Root);
+      win10Roots.push_back(win10Root);
+    }
+  }
+
+  {
     // This logic is taken from the vcvarsqueryregistry.bat file from VS2015
     // Try HKLM and then HKCU.
     std::string win10Root;