Add yet another wxDEPRECATED_XXX macro, this one simply expanding to
C++14 [[deprecated]] attribute if it's available and nothing otherwise.
It's a bit ridiculous to have so many different macros for deprecating
things, but the new one is useful because the standard attribute can be
used to deprecated enum elements, which is impossible with MSVC-specific
__declspec(deprecated).
7a8f2dffbd