The hint is not supported on core profiles. It's not guaranteed to do
anything on compatibility profile either: "The interpretation of hints is
implementation dependent. An implementation may ignore them entirely."

This state was only available on D3D 7 and below (and not guaranteed to
do anything there either, according to older DX SDK docs).
If necessary we could implement it via the GLSL 1.50 "noperspective"
interpolation qualifier, which is what we need to use for the equivalent
D3D10 feature anyway.