write [Paths] to qt.conf only if -external-hostbindir is used

there is no point in overriding the built-in defaults with the same
values.

Change-Id: I24f66b86f751f7044625b5256f3d979ece782cf7
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>