doc: dont try to build manpages if no asciidoc
doc: dont try to build manpages if no asciidoc

jaiminpan/pgbouncer,dbaxa/pgbouncer,digoal/pgbouncer-x2,jaiminpan/pgbouncer,kvap/pgbouncer,kvap/pgbouncer,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,kvap/pgbouncer,wurenny/pgbouncer-x2,dbaxa/pgbouncer,digoal/pgbouncer-x2,dbaxa/pgbouncer,jaiminpan/pgbouncer,wurenny/pgbouncer-x2,wurenny/pgbouncer-x2,digoal/pgbouncer-x2,kvap/pgbouncer,jaiminpan/pgbouncer,dbaxa/pgbouncer