Qt Enterprise for QNX target prefix should be changed

The host prefix (-extprefix) and the target prefix (-prefix) need to be separated.

It's especially problematic for the Windows build because the default prefix contains a colon; i.e., /var/tmp/c:/home/qt/work/install.  This make it impossible to setup Qt in the default location.

For builds that we create, we're using /usr/qt5 as the target prefix.