fix superbuild with ninja
fix superbuild with ninja

Fix the horribly broken PVExternalProject.cmake to actually work with
Ninja. (Note: it works by chance on Windows because it just punts to
plain ExternalProject for that case, but has issues elsewhere.)

frizaro/Veloview,frizaro/Veloview,Kitware/VeloView,frizaro/Veloview,Kitware/VeloView,Kitware/VeloView,Kitware/VeloView,Kitware/VeloView,frizaro/Veloview