Avoid clang -Wnon-virtual-dtor without triggering MSVC warnings

The change of 4990515aba resulted in
(disabled by default, but which may be enabled when building
applications using wxWidgets) MSVS 2017 warning C4265 ('class' : class
has virtual functions, but destructor is not virtual), so test for clang
explicitly instead of just testing for non-gcc.

include/wx/msw/ownerdrawnbutton.h

@@ -42,7 +42,7 @@ protected:
     // even if it is protected, but actually does not give any warnings if the
     // dtor is not defined at all, so work around this 3.4.5 bug inside our
     // general g++ workaround.
-#if !defined(__GNUC__) || wxCHECK_GCC_VERSION(4, 0)
+#if wxCHECK_GCC_VERSION(4, 0) || defined(__clang__)
     ~wxMSWOwnerDrawnButtonBase() { }
 #endif // g++ 4.0+