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-docker.sh

@@ -36,6 +36,9 @@
 set -e # Fail on any error.
 set -x # Display commands being run.
 
+echo "Fetching external projects..."
+./update_glslang_sources.py
+
 echo "Fetching depot_tools..."
 mkdir -p /tmp/depot_tools
 curl https://storage.googleapis.com/chrome-infra/depot_tools.zip -o /tmp/depot_tools.zip