don't link with Octave libs on GNU/Linux

[mtmiller]

Drop rustc-link-lib options on x86_64-unknown-linux-gnu. Duplicate target-dependent link logic into helloworld.

Closes #55.

[mtmiller]

Hmm, I don't really understand the build system. I want to remove the rustc-link-lib options when building oct files, but that breaks cargo test. If I add the options back in octh/build.rs only, they also seem to apply when building oct files. I'm not sure how these pieces fit together. Happy to learn, pointers welcome!

[ctaggart]

May be try the workaround here:
rust-lang/cargo#2549

if cfg!(feature="test") { ... }

And then run with cargo test --features=test.