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The configure scripts, used to generate many of the files in
source/config, have been removed from libaom in favor of cmake scripts.

Modify generate_gni.sh and BUILD.gn to accomodate.

However, the library is not ready to be updated yet, so keep these in
limbo.

Change-Id: I187c7877b3f3a5eadffb3d0055d9e3abf9d17b2a
Reviewed-on: https://chromium-review.googlesource.com/1038437
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Reviewed-by: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#555210}