MinGW compiler predefines WIN32, meaning that wx/msw/winundef.h was
always included from wx/defs.h, even when it was completely unnecessary.
This was just inefficient, but harmless, until the changes of
042d922e88 which broke MinGW compilation
as wxUSE_UNICODE_WINDOWS_H was incorrectly defined during the very first
inclusion of wx/msw/winundef.h, before _UNICODE could be defined
correctly by windows.h.
Fix this by checking whether windows.h was really already included.
9364690c0e