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

Help/variable/CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION.rst

@@ -10,3 +10,8 @@ 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``.  If no Windows 10 SDK
 is available this value will be empty.
+
+One may set a ``CMAKE_WINDOWS_KITS_10_DIR`` *environment variable*
+to an absolute path to tell CMake to look for Windows 10 SDKs in
+a custom location.  The specified directory is expected to contain
+``Include/10.0.*`` directories.

Modules/InstallRequiredSystemLibraries.cmake

@@ -31,6 +31,11 @@
 #   app-local deployment (e.g. to Windows XP).  This is meaningful
 #   only with MSVC from Visual Studio 2015 or higher.
 #
+#   One may set a ``CMAKE_WINDOWS_KITS_10_DIR`` *environment variable*
+#   to an absolute path to tell CMake to look for Windows 10 SDKs in
+#   a custom location.  The specified directory is expected to contain
+#   ``Redist/ucrt/DLLs/*`` directories.
+#
 # ``CMAKE_INSTALL_MFC_LIBRARIES``
 #   Set to TRUE to install the MSVC MFC runtime libraries.
 #
@@ -258,6 +263,7 @@ if(MSVC)
       set(programfilesx86 "ProgramFiles(x86)")
       find_path(WINDOWS_KITS_DIR NAMES Redist/ucrt/DLLs/${CMAKE_MSVC_ARCH}/ucrtbase.dll
         PATHS
+        $ENV{CMAKE_WINDOWS_KITS_10_DIR}
         "${windows_kits_dir}"
         "$ENV{ProgramFiles}/Windows Kits/10"
         "$ENV{${programfilesx86}}/Windows Kits/10"

Source/cmGlobalVisualStudio14Generator.cxx

@@ -219,6 +219,14 @@ std::string cmGlobalVisualStudio14Generator::GetWindows10SDKVersion()
 #if defined(_WIN32) && !defined(__CYGWIN__)
   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.