Before this, -D_WIN32_WINNT=0x0601 used to land to VS project simply
as _WIN32_WINNT (without value).

Change-Id: I68ebdc24c3181c5d9e046280756e4cefee3de46c
Reviewed-on: https://gerrit.libreoffice.org/43928Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>