Properly nest build time headers to match arrangement at install time.

[stellaraccident]

The build tree was missing a level of nesting, causing diversions based on in-tree/out-of-tree use.

[kentrussell]

I've brought this in internally, but am talking to our packaging team to ensure that it doesn't break any current flows. Also I had to modify kfdtest as well, since that broke without the changed paths

[kentrussell]

So this one missed 6.1, but it should be in the 6.2 release.