Update to Buffer Lab

We've uploaded another update to the Buffer Lab to fix a couple of issues.

(1) Some recent gcc builds automatically enable the -fstack-protector option. We now explicitly disable this by compiling the buffer bomb with -fno-stack-protector.

(2) In order to avoid infinite loops during autograding, the previous update from February 2012 introduced a timeout feature that was always enabled. However, this was a problem for students who were debugging their bombs in gdb. We now enable timeouts only during autograding.

Thanks to Prof. James Riely, DePaul University for pointing these out to us.