Build assistant without PrintSupport

Don't skip help and qhelpgenerator when Qt::PrintSupport is not
available, and don't require it for assistant either. Link it only
when present, and guard the printing code with QT_PRINTSUPPORT_LIB,
like Designer does. QT_NO_PRINTER and QT_CONFIG(printer) can't be
used on their own, as they are provided by PrintSupport itself.

Pick-to: 6.12
Task-number: QTBUG-136345
Change-Id: Ie4f76a5a6deafcd3a61a708010a3c8a98915cfda
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>