unit:cpp17: make C++17 test stricter check

many compilers allowed "nodiscard" C++17 feature with pre-c++17 flags.
The C++17 filesystem typically actually does require -std=c++17.
This makes this unit test more representative of C++17 flag support.

test cases/unit/45 vscpp17/main.cpp

@@ -1,7 +1,29 @@
-[[nodiscard]] int foo(void) {
-    return 0;
-}
+#include <iostream>
+
+#if __cpp_lib_filesystem || (defined(__cplusplus) && __cplusplus >= 201703L)
+#include <filesystem>
+#endif
+
+int main(){
+
+#if __cpp_lib_filesystem || (defined(__cplusplus) && __cplusplus >= 201703L)
+char fs = std::filesystem::path::preferred_separator;
+std::cout << "OK: C++17 filesystem enabled" << std::endl;
+#endif
 
-int main(void) {
-    return foo();
+#if defined(_MSC_VER)
+#if _HAS_CXX17
+std::cout << "OK: MSVC has C++17 enabled" << std::endl;
+return EXIT_SUCCESS;
+#else
+std::cerr << "ERROR: MSVC does not have C++17 enabled" << std::endl;
+return EXIT_FAILURE;
+#endif
+#elif defined(__cplusplus) && __cplusplus >= 201703L
+std::cout << "OK: C++17 enabled" << std::endl;
+return EXIT_SUCCESS;
+#else
+std::cerr << "ERROR: C++17 not enabled" << std::endl;
+return EXIT_FAILURE;
+#endif
 }