Turn on parallel builds.

The bugs are believed to be worked out. From a `make clean`, `make` took 8:41
and `make -j6` took 4:43.
auto merge of #902 : metajack/servo/parallel-all-the-builds, r=jdm

The bugs are believed to be worked out. From a `make clean`, `make` took 8:41
and `make -j6` took 4:43.