Fix detection of TROMPELOEIL_HAS_VA_OPT for MSVC

Only the new conformant MSVC preprocessor (_MSVC_TRADITIONAL==0) ever has
__VA_OPT__. This became avaialble with Visual Studio 16.6, but only if
running in /std:c++20 or /std:c++latest mode (i.e. _MSVC_LANG > c++17).
- https://devblogs.microsoft.com/cppblog/announcing-full-support-for-a-c-c-conformant-preprocessor-in-msvc/

Starting with Visual Studio 17.10, it is now enabled regardless of /std
This was changed to better match the behavaior of GCC and clang,
which also loosened up their checks:
- https://developercommunity.visualstudio.com/t/Zc:preprocessor-__VA_OPT__-is-not-enabl/10205604
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98859
  (GCC 13 and up allow __VA_OPT__ in -std=c2x as well as -std=c++20)
- https://reviews.llvm.org/D91913
  (clang 12 and up allow __VA_OPT__ regardless of c/c++ standard)

The MSVC traditional preprocessor (_MSVC_TRADITIONAL defined and nonzero)
never supports __VA_OPT__, regardless of MSVC or c++ standard version.

Author: puetzk

include/trompeloeil/mock.hpp

@@ -122,13 +122,15 @@
 #define TROMPELOEIL_NOT_IMPLEMENTED(...) __VA_ARGS__
 #endif
 
-#if defined(_MSVC_TRADITIONAL) && _MSVC_TRADITIONAL==0
-#  if _MSC_VER >= 1940
-#    define TROMPELOEIL_HAS_VA_OPT 1
-#  else
-#    define TROMPELOEIL_HAS_VA_OPT 0
+#if defined(_MSVC_TRADITIONAL)
+#  if _MSVC_TRADITIONAL==0
+#    if (_MSC_VER >= 1940) || ((_MSVC_VER >= 1926) &&  (_MSVC_LANG > 201703L))
+#      define TROMPELOEIL_HAS_VA_OPT 1
+#    else
+#      define TROMPELOEIL_HAS_VA_OPT 0
+#    endif
+#    define TROMPELOEIL_MSVC_PREPROCESSOR 0
 #  endif
-#  define TROMPELOEIL_MSVC_PREPROCESSOR 0
 #elif __cplusplus >= 202002L
 #  define TROMPELOEIL_HAS_VA_OPT 1
 #else