The MIPSpro 7.4.4 prelinker goes into an infinite loop when it assigns
an instantiation of vtkVariant::ToNumeric<> to an object file such as
"vtkCharArray.o".  It magically loads "vtkVariant.cxx" by guessing the
name based on "vtkVariant.h" in which the template is declared.  Loading
the file causes more instantiations and confuses the prelinker.  Since
we provide explicit instantiations for this method anyway, we hide the
definition from the prelinker using conditional compilation.