Our build logic needed to revised and and clang implements more warnings
than MSVC, so GTest needed more fixes.

Bug: 200
Change-Id: I84c5dd0c51079dd9c990e08dbea7f9022a7d6842
Reviewed-on: https://boringssl-review.googlesource.com/21204
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>