CompileFeatures: Fix hard-coded MSVC C features

In !3226 (merged) 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.