external: adapt tests to Corrade changes.

Now C++17 is properly selected on MSVC, but the __cplusplus macro isn't
doing the right thing there. Use CORRADE_CXX_STANDARD instead.

src/MagnumExternal/Optional/OptionalWrapper.h

@@ -32,7 +32,7 @@
 /* Cover your eyes. This will hurt. And wash your hands after. */
 #ifdef MAGNUM_BUILD_DEPRECATED
 #ifdef __has_include
-#if __has_include(<optional>) && __cplusplus >= 201703L
+#if __has_include(<optional>) && CORRADE_CXX_STANDARD >= 201703L
 #define _MAGNUM_HAS_STD_OPTIONAL
 #include <optional>
 

src/MagnumExternal/Optional/Test/StdOptionalTest.cpp

@@ -50,13 +50,13 @@ StdOptionalTest::StdOptionalTest() {
 #pragma warning(disable: 4996)
 #endif
 void StdOptionalTest::conversion() {
-    Debug{} << "Using C++ version" << __cplusplus;
+    Debug{} << "Using C++ version" << CORRADE_CXX_STANDARD;
     #ifdef _MAGNUM_HAS_STD_OPTIONAL
     Debug{} << "Using a conversion to std::optional, C++17 should be present";
-    CORRADE_VERIFY(__cplusplus >= 201703L);
+    CORRADE_VERIFY(CORRADE_CXX_STANDARD >= 201703L);
     #else
     Debug{} << "Using a typedef to std::optional, C++17 should not be present";
-    CORRADE_VERIFY(__cplusplus < 201703L);
+    CORRADE_VERIFY(CORRADE_CXX_STANDARD < 201703L);
     #endif
 
     Containers::Optional<int> a{5};