Fix cargo 1.55+ build

[Tirka]

Environment variable RUSTFLAGS is not available within build.rs script since 1.55 version of cargo.

Currently, I'm not sure if this is a bug and waiting for clarification.
Tracking issue: rust-lang/cargo#10111.

This Pull Request fixes the problem by using the newly introduced environment variable CARGO_ENCODED_RUSTFLAGS which wraps RUSTFLAGS while keeping backward compatibility.

[darrenldl]

Thanks!

[Tirka]

Thank you for your quick response! Could you publish the updated crate to crates.io?

[darrenldl]

I no longer use this crate (or Rust in general) - could I add you as a repo collaborator and crate owner if you're an active user?