Ship the manpages in the source tarball made by 'make dist', and clean
them up only during the 'make maintainer-clean' step. This allows
distributions not to depend on docbook at lxc build time, because the
manpages are already there.

Also update the configure warning message to sound less scary.
Signed-off-by: Guido Trotter <ultrotter@quaqua.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>