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It defines YY_USE_CONST when building mesa to fix this warning, disables
-Wincompatible-pointer-types for the other third-party libraries that
trigger it and fixes some underspecified dependencies in
third_party/libwebp.

BUG=504696

Review URL: https://codereview.chromium.org/1226583002

Cr-Commit-Position: refs/heads/master@{#338227}