Improve warning for old versions of HPC

I'm still not sure we want to go ahead with the hpc invocation at all if we are discarding search paths, but if we do, then we ought at least warn that we are doing so.

@ttuegel: as the author of bf2fb86dfd5ff40a2a077282d63f8dc40ebaf3b0, if you could comment a bit on whether or not hpc < 0.7 is really even usable, and under what circumstances it's useful, I'll add another commit to this pull request adding comments to the code. Particularly, how likely is it that hpc will still be useful after we discard some of its search paths?