qt: only pass -qt-xcb on non-OS X
qt: only pass -qt-xcb on non-OS X

lgarren/spack,skosukhin/spack,TheTimmy/spack,EmreAtes/spack,skosukhin/spack,tmerrick1/spack,krafczyk/spack,mfherbst/spack,mfherbst/spack,matthiasdiener/spack,lgarren/spack,LLNL/spack,TheTimmy/spack,mfherbst/spack,krafczyk/spack,lgarren/spack,TheTimmy/spack,mfherbst/spack,iulian787/spack,skosukhin/spack,LLNL/spack,mfherbst/spack,lgarren/spack,LLNL/spack,lgarren/spack,tmerrick1/spack,tmerrick1/spack,LLNL/spack,iulian787/spack,matthiasdiener/spack,iulian787/spack,tmerrick1/spack,LLNL/spack,TheTimmy/spack,EmreAtes/spack,EmreAtes/spack,krafczyk/spack,EmreAtes/spack,matthiasdiener/spack,krafczyk/spack,iulian787/spack,krafczyk/spack,TheTimmy/spack,tmerrick1/spack,skosukhin/spack,EmreAtes/spack,iulian787/spack,matthiasdiener/spack,skosukhin/spack,matthiasdiener/spack