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Since libc++{abi} are included using -isystem and we compile with -MMD (not
-MD), headers from these libraries will not be listed as dependencies.  It's
possible for libc++{abi} to be updated and for ninja to belive no rebuild is
necessary.  This CL adds defines for libc++{abi} to force a rebuild when the
headers are updated.

This CL also requires:
https://chromium-review.googlesource.com/c/chromium/buildtools/+/1002869

R=thakis

Change-Id: I4c48aa18643cd5fee1be23b6379569123e6fef51
Reviewed-on: https://chromium-review.googlesource.com/1002868
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550092}