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.
Use the infrastructure added by commit f92ccbc306
(CompileFeatures: memoize C compilers with full language level support)
to avoid using a `try_compile` to check for C 90/99/11 feature support when the running compiler is known to have a fixed set of feature support.
The MSVC C compiler has no notion of C language standards or flags.
Tell CMake to assume that all language standards are available.
Record available C language features depending on the version of
the compiler.
Fixes: #17858
