Clang: MSVC-style C flags

Support added in LLVM commit d087d805acb664e885e9c31a916f6cfa5dbc2186, will be
released in Clang 13.

Modules/Compiler/Clang-C.cmake

@@ -56,10 +56,20 @@ else()
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_C99_STANDARD_COMPILE_OPTION "")
   set(CMAKE_C99_EXTENSION_COMPILE_OPTION "")
-  set(CMAKE_C11_STANDARD_COMPILE_OPTION "")
-  set(CMAKE_C11_EXTENSION_COMPILE_OPTION "")
-  set(CMAKE_C17_STANDARD_COMPILE_OPTION "")
-  set(CMAKE_C17_EXTENSION_COMPILE_OPTION "")
+
+  if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 13.0)
+    set(CMAKE_C11_STANDARD_COMPILE_OPTION "/std:c11")
+    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "/std:c11")
+
+    set(CMAKE_C17_STANDARD_COMPILE_OPTION "/std:c17")
+    set(CMAKE_C17_EXTENSION_COMPILE_OPTION "/std:c17")
+  else()
+    set(CMAKE_C11_STANDARD_COMPILE_OPTION "")
+    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "")
+
+    set(CMAKE_C17_STANDARD_COMPILE_OPTION "")
+    set(CMAKE_C17_EXTENSION_COMPILE_OPTION "")
+  endif()
 endif()
 
 if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 2.1)