WITH_AUTO_OBJ: Fix crunchgen builds.

WITH_AUTO_OBJ: Fix crunchgen builds.

Passing MAKEOBJDIRPREFIX to the main prog build (rescue) would confuse
WITH_AUTO_OBJ and cause it to create a recursed object directory that
then broke the actual prog build.  This is normally not a problem since
we do not call 'make -f prog.mk obj' before building anything in it.

Crunchgen(1) also assumes that if -o is not passed then if an object
directory does not already exist then it should build in the source
directories.  The normal buildworld process will have already ran 'make
obj' in each of the component directories so this is not a problem.
With WITH_AUTO_OBJ though this is not the case.  So we must tell
crunchgen(1) that MK_AUTO_OBJ=yes will create the directory and to not
require it be present before generating its Makefile.

Sponsored by:	EMC / Isilon Storage Division