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If SHELL is not set then make will search PATH for sh.exe. If it is found, it
will use that instead of cmd.exe. If the sh.exe exists in a path with a space,
the command will fail to run.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/806283003

Cr-Commit-Position: refs/heads/master@{#309450}