Also automatically download git submodules with CMake.

Change-Id: I3da095f49d0a9c577ea5dbb50d3db951417f4755
Reviewed-on: https://swiftshader-review.googlesource.com/20088Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>