David Michael authored Nov 04, 2017 and Federico Mena Quintero committed Nov 16, 2017

The primary change here is to add the --target=$(host) option to
the cargo build command, so the Rust components are compiled for
the target host system specified by the configure command.  The
cargo target subdirectory also needed to be prefixed with the host
triplet for cross-compiling.

Some of the crates' build scripts require cross-pkg-config settings
in the environment, so they are set for the cargo build command.

It's worth noting that Rust targets are limited to built-in values
by default, so the --host value given to configure may not be
supported.  Built in targets can be found by listing the directory
src/librustc_back/target in the rustc source.  When building with
an unsupported target, the user will have to write a target JSON
definition file and set the environment variable RUST_TARGET_PATH
to its directory (as with building any Rust project).

This also sneaks in prefixing the Rust library recipe line with a
"+" character to pass the GNU Make jobserver environment and file
descriptors, which cargo supports to act as a jobserver client.