Adding AdditionalOptions to nmake project settings allows IntelliSense
to know about used compiler switches; specifically, passing std switch
(currently -std:c++17) defines _MSVC_LANG macro [1] to proper value to
avoid false IntelliSense errors (e.g., for string_view).

[1] https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros

Change-Id: I90309e6bf2a87257a0c81b1406e678af962eb587
Reviewed-on: https://gerrit.libreoffice.org/67805
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>