A google search on ".hdepend" returned many links, one was this:

http://www.torque.net/kbuild/archive/0048.html

It states in part:

--<begin quote>--

> . . . If you interrupt a 'make dep' at the 
> right point, you wind up with an invalid .hdepend that needs to be 
> removed. 

Aha, the generic way to fix that is: 

    dep-files: 
        scripts/mkdep `find $(HPATH) ...` > .hdepend.new 
        mv .hdepend.new .hdepend 

--<end quote>--

Looks benign enough to try.  I will give it a go this evening, inspecting
the .hdepend.new file before I mv it.  Does it make sense to you?

Alan