Fix parallel build failures

This is the 2.5 version of the "fix build failures":

- man/%.gen could be left over even in case of failure, due to
  automake bug
- make man/%.gen runs RUN_IN_TEMPDIR, so let's depend on it, since
  that target has the proper dependencies (create needed dirs)
- man/%.gen depends on a number of built sources, but the dependency
  was not declared

Furthermore, wraps a long comment.

Tested with -j4/-j16, after `make maintainer-clean'.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>