The .hhc file is generated but it is named with the base name of the source file
instead of toc.
This change was made in 8.5.0 (2009-10-04).
But I uncovered a related regression: the naming of the .hhp file had reverted
to the default in 8.6.2 (r881). I've fixed this in the trunk and made the .hhk
file name follow the same convention: