Make cmake submodule update work with older versions of git

[johnomotani]

In older versions of git submodule update --init --recursive had to be run from the root of the repo. This PR replaces CMAKE_CURRENT_SOURCE_DIR with PROJECT_SOURCE_DIR in the cmake call to update the submodules to ensure that the update runs from the repo root, making it compatible with older git versions.

Before this fix, a CMake build with a too-old version of git would error with a message like

You need to run this command from the toplevel of the working tree.
CMake Error at CMakeLists.txt:47 (message):
  git submodule update --init failed with 1, please checkout submodules
Call Stack (most recent call first):
  tests/unit/CMakeLists.txt:1 (bout_update_submodules)


-- Configuring incomplete, errors occurred!