The CMake flag and define set in Configure.h.in differ in their capitalization of the 'm' in VTKm so I've changed the CMake flags to match those used in Configure.h.in. I'm not sure if fixing it the other way around might be preferred to keep all the CMake flags prefixed with VTKm instead of mixing VTKm/VTKM.
This fix also revealed some methods in CellSet and CellSetExplicit where ints where used instead of vtkm::Ids which caused errors with child classes who overrided methods and returned a vtkm::Id instead of an int.
Also fixed issues that appeared in TestOutOfMemory which got out of date due
to not being compiled since the VTKM_USE_64BIT_IDS flag would never be set.
The test now runs and passes when 64bit ids are enabled.
Let me know what you think, I wasn't sure which way to change the flag/define names to match convention.