CompileFeatures: Fix hard-coded MSVC C features

In commit 8e4899fd6c (CompileFeatures: Record which C features the MSVC
compiler supports, 2019-04-12) our `cmake_record_c_compile_features`
macro was accidentally left not setting the `_result` variable, which
had previously been set by `_record_compiler_features`.  The variable is
expected by the call site in `cmake_determine_compile_features` and used
to switch between "failed" and "done" reports.  Set it now.

Also record `c_variadic_macros` only for cl 14 (VS 2005) and higher
because it is not supported before that version.

Modules/Compiler/MSVC-C.cmake

@@ -21,9 +21,13 @@ macro(cmake_record_c_compile_features)
     c_std_99
     c_std_11
     c_function_prototypes
-    c_variadic_macros
     )
   list(APPEND CMAKE_C90_COMPILE_FEATURES c_std_90 c_function_prototypes)
-  list(APPEND CMAKE_C99_COMPILE_FEATURES c_std_99 c_variadic_macros)
+  list(APPEND CMAKE_C99_COMPILE_FEATURES c_std_99)
   list(APPEND CMAKE_C11_COMPILE_FEATURES c_std_11)
+  if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 14.0)
+    list(APPEND CMAKE_C_COMPILE_FEATURES c_variadic_macros)
+    list(APPEND CMAKE_C99_COMPILE_FEATURES c_variadic_macros)
+  endif()
+  set(_result 0) # expected by cmake_determine_compile_features
 endmacro()