doxygen has been failing for a while now on -CURRENT and apparently
-STABLE too. The current fix is disabling one of the tests in the
build, but obviously it points to a problem with our base system....
I've trussed  the failing code , and it looks as though it's
hanging on a _umtx call. I'm gratuitously ignorant of what goes on
there... but the timings of recent commits to umtx.h  could
indicate a link (hope it's not bogus...).