Permit VTK_WRAP customisation
The __VTK_WRAP__
compiler directive in VTK 8.1 introduces hard-coded C++11 style attributes which are otherwise blank. A small change the to conditional definitions permits a dependent application/library to specify its own attributes. This is necessary for the Clang compiler, which ignores the attributes; vtk::newinstance, vtk::zerocopy, vtk::expects(x), vtk::sizehint(VA_ARGS).