Bug 863091 - Make auto clobbering opt-in rather than opt-out; r=glandium

To avoid surprises for devs building locally, auto clobbering is now opt-in, via
the use of |mk_add_options AUTOCLOBBER=1| in the mozconfig. Our automation uses
build/mozconfig.common which specifies this, so auto clobber will remain enabled
for our buildbot machines.