jobserver doesn't always work

[BusyJay]

When using cmake-rs to build a native library, I can see some errors on some platform:

make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

The problem may be related to rust-lang/jobserver-rs#47.

This is not an issue with cmake-rs, but cmake-rs can let user to disable jobserver explicitly to get around the problem.

And tikv/grpc-rs# also provides a workaround by setting environment variable CMAKE_BUILD_PARALLEL_LEVEL. And using generator other than make can also get around the problem (setting environment variable MAKE_GENERATOR=Ninja for example).

[Notgnoshi]

I wonder if this is related to #172?

I expect #172 is caused by the caveat mentioned in https://docs.rs/jobserver/latest/jobserver/struct.Client.html#method.from_env about from_env being unsafe to call twice in the same process if the first invocation returned Some.