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.
This allows glslang to be build standalone using the gn build system.
To build with gn:
```
gclient sync --gclientfile=standalone.gclient
gn gen out/default
cd out/default
ninja
```