This is to be used in place of BOOST_STATIC_ASSERT so that we can
control its implementation.

The implementation is designed to fix the issue where the latest XCode
clang compiler gives a warning about a unused typedefs when the boost
static assert is used within a function. (This warning also happens when
using the C++11 static_assert keyword.) You can suppress this warning
with _Pragma commands, but _Pragma commands inside a block is not
supported in GCC. The implementation of VTKM_STATIC_ASSERT handles all
current cases.