Use metadeps to specify pkg-config dependencies declaratively

[joshtriplett]

This makes it easier for distribution packaging tools to generate
appropriate package dependencies.

[sfackler]

Neat! It looks like this will bump the minimum supported Rust version to 1.13 due to the error-chain dependency.

[joshtriplett]

@sfackler Interesting; I didn't realize that error-chain required 1.13. I reported that as rust-lang-deprecated/error-chain#101 .

If that issue gets addressed, hopefully everything else will work with Rust 1.10. If it doesn't, I may abandon error-chain in metadeps. (I like it, but I already think I may disable the backtrace feature that pulls in many dependencies. If it also prevents widespread use by projects that need to run on older Rust, then I'll have to drop it.)

[joshtriplett]

@sfackler OK, I've gotten a change into error-chain so it works with Rust 1.10, and updated metadeps to work with that. As soon as error-chain 0.7.2 gets released to crates.io, I'll re-push this so it re-runs CI and confirms the functional build on Rust 1.10.

[sfackler]

Looks like that was published and the builds are happy. Thanks!