Specify that the ${GENERATED_HELP_TAGS} "command" (output) depends on
`helptags` so that it always regenerates the doc/ tags. (cmake "targets"
always run, whereas "commands" are contingent on their dependencies. But
we don't define doc/ dependencies because they are circular.)
Declare dependency in terms of directory, rather than individual doc
files to avoid target dependency cycles. This still maintains install
targets at doc file level.
I see that problem fixed by #2801 was resurrected by making help tags file
generated in a more direct way. This fixes the hang without using the empty
file.
