CMakeCCompilerId: Fix C standard detection in Clang MSVC mode

Clang doesn't define __STDC__ if in MSVC compatibility mode, but does define
__STDC_VERSION__.
Avoid the fallback for this combination.

Modules/CMakeCCompilerId.c.in

@@ -33,9 +33,8 @@ char const *info_cray = "INFO" ":" "compiler_wrapper[CrayPrgEnv]";
 @CMAKE_C_COMPILER_ID_PLATFORM_CONTENT@
 @CMAKE_C_COMPILER_ID_ERROR_FOR_TEST@
 
-#if !defined(__STDC__)
-# if (defined(_MSC_VER) && !defined(__clang__)) \
-  || (defined(__ibmxl__) || defined(__IBMC__))
+#if !defined(__STDC__) && !defined(__clang__)
+# if defined(_MSC_VER) || defined(__ibmxl__) || defined(__IBMC__)
 #  define C_DIALECT "90"
 # else
 #  define C_DIALECT

Tests/RunCMake/CMakeLists.txt

@@ -417,6 +417,13 @@ function(add_RunCMake_test_try_compile)
       set(CMAKE_C_STANDARD_DEFAULT "")
     endif()
   endif()
+  if(CMAKE_VERSION VERSION_LESS 3.20.20210225 AND "x${CMAKE_C_COMPILER_ID}" STREQUAL "xClang")
+    # Older CMake versions do not know about Clang MSVC compatibility mode
+    # standards. Approximate the logic from Clang-C.cmake.
+    if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 3.5.2)
+      set(CMAKE_C_STANDARD_DEFAULT 11)
+    endif()
+  endif()
   foreach(var
       CMAKE_SYSTEM_NAME
       CMAKE_C_COMPILER_ID