Allow using libjit-sys with Rust 1.0

[emk]

This will provide low-level access to libjit-sys to people using Rust 1.0. For the fun, high-level stuff, they'll still need to switch to the nightly branch. :-)

To do this, I needed to do two things:

• Use the cargo-ized version of libc, because the in-compiler version probably won't be un-feature-gated in the immediate future.
• Replace PathExt::exists with a Rust-1.0-compatible implementation.

I tested this using multirust:

cd jit.rs/sys
multirust override stable
cargo test

...and I tested the main library as usual using the nightly build:

cd ..
cargo test

[emk]

Thank you for the merge! I also have a unit test and an example program for sys which will be along in a minute.