The VTKM_MSVC is just defined if using MSVC. It is not set to any value.
There was a reference in Math.h that assumed it was set to true, which
it was not. This simple change should fix that.
Also fix some issues that caused the compile to fail when trying to
run some of the math functions on a CUDA device. In particular, CUDA
is picky about using a global const on a device when the const type is
not one of the basic C types.
