rust: function macro now returns the function name

rust: function macro now returns the function name

Borrow a macro from https://github.com/popzxc/stdext-rs that
will give us the Rust function name in SCLog messages in Rust.

As this trick only works on Rust 1.38 and newer, keep the old
macro around and set a feature based on a Rust version test
done during ./configure.