RTFM - at least the one which comes with the distribution. I think the online one does not have OpenSolaris support. And, I am right it does not.
Anyway, you do not need to retrieve the qt libraries and build, as I just did, it is supplied for you.
cd /opt/VirtualBox LD_LIBRARY_PATH=/opt/VirtualBox:/opt/VirtualBox/qtgcc/lib:. ./VirtualBox