cargo features for quickchecking crate

[shnewto]

The quickchecking crate, for property testing with fuzzed C headers, has some special casing so that known issues (#550, #684, and #1153) don't cause the property tests to fail. That special casing could be handled with a feature in the crate's Cargo.toml file and some cfg attributes in the source.

Corresponds to extending #970

quickchecking crate lives in the tests directory here: https://github.com/rust-lang-nursery/rust-bindgen/tree/master/tests/quickchecking