We now have gn flags to request unbundling of zlib and lcms2 in pdfium
and can add those to our configure system.

Task-number: QTBUG-61746
Change-Id: Iccc4c2a09b7b99121d79348f9e0636785b29f503
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>