static libraries not reproducible on Mac, causes Rust cache misses

[luser]

@metajack found this while trying to use sccache to build servo on Mac. There's a crate that uses the gcc crate in a build script to build a static library, and the crate is always a cache miss because sccache includes the hash of the static library in the cache key, but Apple's ld includes timestamps in static libraries (with no way to disable that behavior, AFAICT), so the file never hashes the same.

We discussed a few remedies, including having the gcc crate zero out the timestamps on static libraries it generates, or having sccache parse static libraries and hashing only the object files within.

[froydnj]

rust-lang/cc-rs#225 was added to make Mac ar invocations more deterministic, but apparently that didn't address all of the issues. Modifying sccache's hashing of ar files seems like it's going to be necessary.

[metajack]

I have a WIP patch for this: https://github.com/metajack/sccache/tree/custom-hashing