Clang: Fix typo to enable compiler-generated Makefile dependencies on Windows

Fix a typo from commit 2c71d051fa (Makefiles Generators: use compiler
for dependencies generation, 2020-10-18, v3.20.0-rc1~392^2).

Modules/Platform/Windows-Clang-C.cmake

@@ -1,7 +1,7 @@
 include(Platform/Windows-Clang)
 __windows_compiler_clang(C)
 
-if("x${MAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
+if("x${CMAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
   if((NOT DEFINED CMAKE_DEPENDS_USE_COMPILER OR CMAKE_DEPENDS_USE_COMPILER)
       AND CMAKE_GENERATOR MATCHES "Makefiles|WMake"
       AND CMAKE_DEPFILE_FLAGS_C)

Modules/Platform/Windows-Clang-CXX.cmake

@@ -2,7 +2,7 @@ include(Platform/Windows-Clang)
 set(_COMPILE_CXX_MSVC " -TP")
 __windows_compiler_clang(CXX)
 
-if("x${MAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
+if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
   if((NOT DEFINED CMAKE_DEPENDS_USE_COMPILER OR CMAKE_DEPENDS_USE_COMPILER)
       AND CMAKE_GENERATOR MATCHES "Makefiles|WMake"
       AND CMAKE_DEPFILE_FLAGS_CXX)