makeparallel: reset make's unlimited stack

make 3.81 sets its own stack size to be unlimited, but accidentally
leaves it unlimited for all child processes.  If it is unlimited, reset
it back to a reasonable default (8MB).

See http://savannah.gnu.org/bugs/?22010

Change-Id: Ieb0289823f12a421b59d8ab5292d3df3c6dfc27e