fix `rewrite_includes_only = false` clang failure
fix `rewrite_includes_only = false` clang failure

The option is called `-x cpp-output` not `-x c-cpp-output`.

Fixes #651.

mozilla/sccache,glandium/sccache,luser/sccache2,luser/sccache2,glandium/sccache,luser/sccache2,mozilla/sccache,luser/sccache2,glandium/sccache,mozilla/sccache,glandium/sccache