Client::configure can't be used to call e.g. make

[glandium]

Client::configure only sets CARGO_MAKEFLAGS, and there is no way to retrieve the value it sets in the Command because there is no API for that. So it's not possible to set MAKEFLAGS yourself to pass cargo's jobserver to GNU make in a build script.

[alexcrichton]

This was changed in 6ac9e7c I think for compatibility reasons or maybe avoiding breakage (I forget at this point...), but adding an option to set MAKEFLAGS seems reasonable to me!

[est31]

The cargo docs added by commit rust-lang/cargo@cbf25a9 aren't really accurate because of this issue:

* `NUM_JOBS` - the parallelism specified as the top-level parallelism. This can
               be useful to pass a `-j` parameter to a system like `make`. Note
               that care should be taken when interpreting this environment
               variable. For historical purposes this is still provided but
               recent versions of Cargo, for example, do not need to run `make
               -j` as it'll automatically happen. Cargo implements its own
               [jobserver] and will allow build scripts to inherit this
               information, so programs compatible with GNU make jobservers will
               already have appropriately configured parallelism.

The jobserver implemented/provided by cargo to build scripts is not compatible with make jobservers. So you need to absolutely use NUM_JOBS right now to pass on the -j param, or copy over the "CARGO_MAKEFLAGS" env var to "MAKEFLAGS" so that cargo understands it...

IMO cargo should just set MAKEFLAGS.

Right now it seems that the jobserver crate only provides compatibility with make when make is the master, but not when make is the slave and the tool that uses this crate is the master.

[est31]

I guess the main issue why @alexcrichton made 6ac9e7c is to allow outside MAKEFLAGS to get to sub-makes, which I agree is an important concern. However, I think some smart appending should be able to do it, e.g. one that parses the present MAKEFLAGS env var, removes any exant params that specify a jobserver, and re-adds the jobserver params of cargo.