Fix GN build and presubmits

Add missing `.cfg` files for GN presubmit.
Add missing `recursedeps` in the `DEPS` file.
Call `./update_glslang_sources.py` before attempting to build.
Add more GN spew to the `.gitignore` file.

kokoro/linux-clang-gn/build.sh

@@ -43,6 +43,7 @@ docker run --rm -i \
   --workdir "${ROOT_DIR}" \
   --env ROOT_DIR="${ROOT_DIR}" \
   --env SCRIPT_DIR="${SCRIPT_DIR}" \
-  --env BUILD_SHARED_LIBS="${BUILD_SHARED_LIBS:-0}" \
   --entrypoint "${SCRIPT_DIR}/build-docker.sh" \
   "gcr.io/shaderc-build/radial-build:latest"
+
+sudo chown -R "$(id -u):$(id -g)" "${ROOT_DIR}"