write all properties to qt.conf when -external-hostbindir is used

the presence of a [Paths] section causes QLibraryInfo to derive all
property values according to the Qt default directory layout,
disregarding the compiled-in paths from configure. consequently, we need
to write them all to qt.conf as well.

Change-Id: I3558e9aef1fce956812ea91e216f53bf7934c285
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>