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Add a GNU make jobserver implementation to Cargo
This commit adds a GNU make jobserver implementation to Cargo, both as a client of existing jobservers and also a creator of new jobservers. The jobserver is actually just an IPC semaphore which manifests itself as a pipe with N bytes of tokens on Unix and a literal IPC semaphore on Windows. The rough protocol is then if you want to run a job you read acquire the semaphore (read a byte on Unix or wait on the semaphore on Windows) and then you release it when you're done. All the hairy details of the jobserver implementation are housed in the `jobserver` crate on crates.io instead of Cargo. This should hopefully make it much easier for the compiler to also share a jobserver implementation eventually. The main tricky bit here is that on Unix and Windows acquiring a jobserver token will block the calling thread. We need to either way for a running job to exit or to acquire a new token when we want to spawn a new job. To handle this the current implementation spawns a helper thread that does the blocking and sends a message back to Cargo when it receives a token. It's a little trickier with shutting down this thread gracefully as well but more details can be found in the `jobserver` crate. Unfortunately crates are unlikely to see an immediate benefit of this once implemented. Most crates are run with a manual `make -jN` and this overrides the jobserver in the environment, creating a new jobserver in the sub-make. If the `-jN` argument is removed, however, then `make` will share Cargo's jobserver and properly limit parallelism. Closes rust-lang#1744
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