Avoid brittle wcscpy_s and wcscat_s defines on mingw

[mstorsjo]

These wcscpy_s and wcscat_s fallback defines seem to be inteded for older versions of MSVC. When building in mingw mode, _MSC_VER isn't defined at all.

Previously this led to these defines being used, which was harmless at time time.

After mingw-w64/mingw-w64@9dff64a in mingw-w64, these fallback defines break using mingw-w64 headers; the expansion of wcscpy_s into two separate statements breaks use of wcscpy_s function in an expression context, leading to errors like this:

In file included from /home/martin/code/libvpl/libvpl/src/windows/mfx_dispatcher.cpp:14:
In file included from /home/martin/clang-nightly/x86_64-w64-mingw32/include/windows.h:114:
/home/martin/clang-nightly/x86_64-w64-mingw32/include/stralign.h:208:67: error: expected ')'
  208 |     if(WSTR_ALIGNED(Source) && WSTR_ALIGNED(Destination)) return (wcscpy_s((PWSTR)Destination,DestinationSize,(PCWSTR)Source)==0 ? Destination : NULL);
      |                                                                   ^
/home/martin/code/libvpl/libvpl/src/windows/mfx_dispatcher_defs.h:22:24: note: expanded from macro 'wcscpy_s'
   22 |         (void)(to_size);                \
      |                        ^
/home/martin/clang-nightly/x86_64-w64-mingw32/include/stralign.h:208:66: note: to match this '('
  208 |     if(WSTR_ALIGNED(Source) && WSTR_ALIGNED(Destination)) return (wcscpy_s((PWSTR)Destination,DestinationSize,(PCWSTR)Source)==0 ? Destination : NULL);
      |                                                                  ^
/home/martin/clang-nightly/x86_64-w64-mingw32/include/stralign.h:208:67: error: cannot initialize return object of type 'PUWSTR' (aka 'wchar_t *') with an rvalue of type 'void'
  208 |     if(WSTR_ALIGNED(Source) && WSTR_ALIGNED(Destination)) return (wcscpy_s((PWSTR)Destination,DestinationSize,(PCWSTR)Source)==0 ? Destination : NULL);
      |                                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/martin/code/libvpl/libvpl/src/windows/mfx_dispatcher_defs.h:22:9: note: expanded from macro 'wcscpy_s'
   22 |         (void)(to_size);                \
      |         ^~~~~~~~~~~~~~~
In file included from /home/martin/code/libvpl/libvpl/src/windows/mfx_dispatcher.cpp:14:
In file included from /home/martin/clang-nightly/x86_64-w64-mingw32/include/windows.h:114:
/home/martin/clang-nightly/x86_64-w64-mingw32/include/stralign.h:208:150: error: extraneous ')' before ';'
  208 |     if(WSTR_ALIGNED(Source) && WSTR_ALIGNED(Destination)) return (wcscpy_s((PWSTR)Destination,DestinationSize,(PCWSTR)Source)==0 ? Destination : NULL);
      |                                                                                                                                                      ^
3 errors generated.

Avoid this issue by simply limiting the fallback wcscpy_s and wcscat_s defines to older MSVC versions, which seems to be the original intent, avoiding defining them on mingw targets, that don't need them.