Make Uniscribe optional on nmake win32 builds

Make Uniscribe optional on nmake win32 builds

Most of harfbuzz clients don't need Uniscribe and DirectWrite support at all
as harfbuzz is a replacement for Uniscribe so Chromium and Firefox are using
harfbuzz without them on Windows.

This removal also helps win32 nmake clients to have reduced binary size and
libs count.