Support selecting a different Rust toolchain
At the moment the values passed through the RUSTC and CARGO environment
variables when running configure are ignored and just rustc
/cargo
as found on the PATH are used.
This is problematic during package builds where we might have to select a specific Rust toolchain, i.e., make sure to always use the system Rust toolchain vs a rustup toolchain.
Signed-off-by: Tobias Kortkamp tobik@FreeBSD.org