Fix the .MAKE added in r251750 to properly support the historical -n -n.

The condition used matches the condition in sys.mk for setting _+_ to blank
or +.

With this -n will continue to not descend into Makefile.inc1, while -n -n will
and cause Makefile.inc1's target to run with -n.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division