Add ant task to html manual and man page generation:

- check-man checks for the presence of the "man"
  directory and sets an ant property skip-man in
  case the directory is missing
- all the man age generating tasks first call
  check-man as a dependency, and then only run
  if skip-man is unset.

Thats useful for unbundled modules like mod_ftp
and mod_fcgid, which do not have a man directory.
Building their docs otherwise bombs.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@816938 13f79535-47bb-0310-9956-ffa450edef68