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This tells the compiler to not look in various env vars (e.g. %INCLUDE%)
which makes the build slightly more hermetic.

(MSVC still uses the env files to get at %INCLUDE% so we can't pass /X to cl,
but clang-cl gets all system include dirs passed as -imsvc flags instead.)

Also move /utf-8 from the warnings config to the compiler config as it's
not a warning flag.

Bug: 691286
Change-Id: I5306d33bbfacd03a267b9d1c082297a26aadf9b8
Reviewed-on: https://chromium-review.googlesource.com/993552Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548030}