Added support for pkg-config

[golddranks]

Supporting pkg-config will allow linking a whole bunch of native dependencies flexibly without specifying link flags for each and every one separately. Some *-sys crates support it in their build.rs scripts, but sadly not all. Trying to improve the situation bit by bit.

[sgrif]

This appears to have broken 3ad9f7b, and I don't see an easy way to fix it. Ultimately the problem is that given the setup that cause the problem that commit fixed in the first place, pkg_config will end up putting /usr/local/lib ahead of the native search path, breaking everything.

I'm honestly not entirely sure what this actually adds over what was there before, either. However, I don't want to flat revert just yet, so I'm going to put it behind a feature flag.

[golddranks]

Indeed, it seems that there isn't an easy way to fix it.

Btw. I did what I could: I posted a suggestion to the pkg-config crate to do the symlink-following thing (since that leads to the single-library-exclusive dirs which prevent conflicts. This works at least in the case of Homebrew, but I think some other packaging systems do this kind of namespacing too, so it should be generally a beneficial thing to do.) If they do that, it should prevent passing /usr/local/lib to Cargo. Here's the issue: rust-lang/pkg-config-rs#34

Also, I noticed that Homebrew .pc files were inconsistent: some declared the lib dir to be /usr/local/lib, and some declared directly the "private" install dir. The latter obviously causes less conflicts, so I sent an issue to Homebrew too. (However, I'm quite oblivious to their system so I don't know if it's going to help at all) Homebrew/homebrew-core#8472