Follow up to a6675a5f, make similar changes in .gn files

- Disable 4996 in GN build
- Fix lib dir for 8.1 sdk.

R=jam@chromium.org
TBR=brettw@chromium.org
BUG=432748

Review URL: https://codereview.chromium.org/730263002

Cr-Commit-Position: refs/heads/master@{#304514}