Fix makeman creating obj directories due to turning on WITH_AUTO_OBJ.

r284708 addressed this slightly but seems to have put the make(showconfig)
guard in the wrong place. Rather than guard setting the default obj directory,
guard inclusion of auto.obj.mk. This avoids creating SRCTOP/obj and
SRCTOP/release/obj when running makeman.

Sponsored by: EMC / Isilon Storage Division