Verify that we only scan generated .h files once.
This originated as a real-life bug report submitted by Scott Lystig
-Fritchie. It's been left as-is, rather than stripped down to bear
+Fritchie. It's been left as-is, rather than stripped down to bare
minimum, partly because it wasn't completely clear what combination of
factors triggered the bug Scott saw, and partly because the real-world
complexity is valuable in its own right.