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.
diff --git a/DEPS b/DEPS
index 9c0e045..2f5fa8e 100644
--- a/DEPS
+++ b/DEPS
@@ -75,3 +75,8 @@
     'condition': 'not build_with_chromium',
   },
 ]
+
+recursedeps = [
+  # buildtools provides clang_format, libc++, and libc++abi
+  'buildtools',
+]