Skip 'update' step for external dependencies

We check them out from a git tag, so this *should* be a noop. However,
cmake seems to want to rebuild these dependencies every time you run
make as it assumes the dependency *might* have been updated. (In
practice, this isn't completely awful, as make notices the files haven't
changed, so files don't actually get recompiled. But the configure step
is still re-run and all the files still need to be rescanned.)

Skipping the update step speeds up the build considerably.

On my linux box, running:
  cmake .. && make -j && time make -j
takes ~8.5s prior to this CL and ~6.5 afterwards. (6s is used by the
test suite.) The upcoming protobuf addition would otherwise have made
this much worse. (It takes a long time to ./configure.)

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