Don't apply clang flag to gcc builds.

For whatever reason GCC doesn't fail the build on unknown flags unless
there is also a warning emitted. Since we're now deprecating
UniquePtr, the warning is thrown here and the build fails (despite not
being -Werror).

Bug: http://b/22403888
Change-Id: I4af7911a51f7af9aa47a596a76f3e9d79535585b