Since `clang-4.0` is no longer being used, it should be safe to remove use of this diagnostic for clang. This enables using `gcc` as a workaround for `g-ir-scanner`, as `unused-command-line-argument` is not recognized by `gcc`. See: https://github.com/ostreedev/ostree/pull/1871#issuecomment-503765624 Closes: #1871 Approved by: jlebon