sccache doesn't handle "--"

[glandium]

rust-lang/cc-rs@fcb8df5 added "--" to the command line emitted for clang-cl when compiling through the cc-rs crate, and sccache doesn't handle it, so it passes it to the preprocessor without considering its effect, and every argument that comes after it on the preprocessor command line is considered as a file name, which, when they are originally flags, well, don't match any actual file, and then the preprocessor fails.