cmake --build . --target install not supported

[Leengit]

By default cmake's ExternalProject_Add command assumes that a dependency like ospray will install with a cmake --build . --target install command. This command doesn't work with ospray (at least when using -GNinja), but I request that it should. To get ExternalProject_Add(ospray, ...) to work, I had to instead add a INSTALL_COMMAND cmake --install . parameter and value. While cmake --install . is otherwise a reasonable alternative, it doesn't work with default behavior of ExternalProject_Add(ospray, ...).