Add C++11 flags to sub-projects
Recently VTK-m was changed to require C++11. The internal builds set properties to require C++11, but these never make it to the configuration for projects that use the VTK-m package (i.e. not declared in VTKmConfig.cmake).
This change adds a new CMake variable to the VTK-m package, VTKm_TARGET_PROPERTIES, which sets the target properties necessary for C ++11.