Handle cases when manpages gzipped before install

Change-Id: Ifeb9bbfc64087595898d3b1af64767edd702ea4f
Reviewed-on: http://review.couchbase.org/23770
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>