When adding sccache toolchain jobs in bug 1381772, building with gcc failed, and building with clang worked, so I just went with the path of least resistance. That's however a suboptimal position in the dependency graph, so it's still preferable to use gcc if possible. Looking exactly how it fails, it turns out it's because without CC being set, ring wants to build with "cc", which ends up being the system gcc instead of ours (our gcc archive doesn't provide "cc", only "gcc"), and it is too old to support the compiler flags ring uses. So setting CC does the trick. Review commit: https://reviewboard.mozilla.org/r/189024/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/189024/