doc/man: only require asciidoc-attrs.conf when building the man pages

Situations:

* If you want to and can build the man pages:
  * If it's a tarball tree:
    * Make the man page destinations depend on asciidoc-attrs.conf.
      Since it's a generated file, its date is greater than the
      date of the prebuilt man pages, therefore the man pages are
      built again, which is a good thing because they include the
      default values of this build.
  * If it's a Git tree:
    * Always build the man pages anyway (no prebuilt man pages here).
* If you want to, but cannot build the man pages:
  * If it's a tarball tree:
    * Make the man page destinations NOT depend on asciidoc-attrs.conf,
      because its recent date would ask said destinations to be rebuilt
      and this is not possible because we don't have the tools.

      However, warn the user at configure time that the prebuilt man
      pages will be installed, which means that they will contain the
      project's default values, not this build's default values.
  * If it's a Git tree:
    * Not valid: error at configure time as usual.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>