MSVC: Exclude future cl 20+ from MSVC14 variable

Modules/Platform/Windows-MSVC.cmake

@@ -78,7 +78,9 @@ if(NOT MSVC_VERSION)
   set(MSVC90)
   set(CMAKE_COMPILER_2005)
   set(CMAKE_COMPILER_SUPPORTS_PDBTYPE)
-  if(NOT "${_compiler_version}" VERSION_LESS 19)
+  if(NOT "${_compiler_version}" VERSION_LESS 20)
+    # We no longer provide per-version variables.  Use MSVC_VERSION instead.
+  elseif(NOT "${_compiler_version}" VERSION_LESS 19)
     set(MSVC14 1)
   elseif(NOT "${_compiler_version}" VERSION_LESS 18)
     set(MSVC12 1)