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Update bazel WORKSPACE and BUILD files to work better on Windows. (#581)

Note, bazel only supports MSVC on Windows, and not MinGW, so
linking against shlwapi.lib only needs to follow MSVC conventions.

git_repository() did not work in local testing, so is swapped for
http_archive(). The latter is also documented as the preferred way
to depend on an external library in bazel.