Add wxCHECK_CXX_STD() and use it to test for C++17 and C++20

Unlike direct tests of __cplusplus, using this macro also works with
(recent enough, i.e. MSVS 2015.3 or later) MSVC versions, even if
/Zc:__cplusplus is not used.

This simplifies some checks and makes some other ones (notably the check
for C++20 used before wxALLOW_COMBINING_ENUMS macro definition) work
with MSVC versions with C++20 support as intended.

tests/test.cpp

@@ -133,7 +133,7 @@ static void TestAssertHandler(const wxString& file,
         // so we'd just die without any useful information -- abort instead.
         abortReason << assertMessage << wxASCII_STR(" in a worker thread.");
     }
-#if __cplusplus >= 201703L || wxCHECK_VISUALC_VERSION(14)
+#if wxCHECK_CXX_STD(201703L)
     else if ( uncaught_exceptions() )
 #else
     else if ( uncaught_exception() )