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To allow embedding SPIRV-Tools in multiple projects (Chromium, ANGLE,
Skia, Dawn) we added overridable variables that replace hardcoded paths
to dependencies.

BUG=chromium:870747

Change-Id: I2041528305739035b46c700719386597bd9b553a
Reviewed-on: https://chromium-review.googlesource.com/1162239Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581252}